Magnesium oxide-polyamine adsorbent and a method of capturing carbon dioxide

ABSTRACT

An aminated magnesium oxide adsorbent containing a magnesium oxide matrix having disordered mesopores and a BET surface area of 320 to 380 m 2 /g, and a polyamine selected from the group consisting of an ethyleneamine having a molecular weight of up to 450 g/mol and a polyethylene imine having a number average molecular weight of greater than 500 g/mol and up to 20,000 g/mol, wherein the polyamine is impregnated within the disordered mesopores of the magnesium oxide matrix. A method of making the aminated magnesium oxide adsorbent and a method of capturing CO 2  from a gas mixture with the aminated magnesium oxide adsorbent are also described.

STATEMENT OF ACKNOWLEDGEMENT

This work was supported by the Deanship of Scientific Research (DSR) atKing Fahd University of Petroleum and Minerals (KFUPM) in the terms ofInternal Research Grant #IN151020.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to an aminated magnesium oxide adsorbent,particularly a magnesium oxide-polyamine adsorbent, methods of makingthe aminated magnesium oxide adsorbent, and methods of capturing carbondioxide (CO₂) with the aminated magnesium oxide adsorbent.

Discussion of the Background

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly nor impliedly admitted as prior art against the presentinvention.

Global warming is a serious problem facing human civilization. Anincrease in the average Earth temperature by even a few degrees mightlead to the melting of the polar ice caps, a significant increase in thewater level of oceans and seas and, thus, the submerging of some islandsand coastal cities. The predicted increase in the average Earthtemperature in this century is about 1.5° C. if the emissions ofgreenhouse gases continue at their current rates. See G. Gómez-Pozuelo,E. S. Sanz-Pérez, A. Arencibia, P. Pizarro, R. Sanz, D. P. Serrano, CO₂adsorption on amine-functionalized clays, Microporous and MesoporousMaterials, 282 (2019) 38-47, incorporated herein by reference in itsentirety. Despite these consequences, carbon dioxide (CO₂) emissionscontinue to rise. The current CO₂ concentration in the atmosphere isabout 410 ppm compared to about 300 ppm in the middle of the twentiethcentury. The average annual increase in the atmospheric level of CO₂ inthe past few decades has been estimated to be about 2 ppm, which is analarming CO₂ accumulation rate in the atmosphere. See M. Fasihi, O.Efimova, C. Breyer, Techno-economic assessment of CO₂ direct air captureplants, Journal of Cleaner Production, 224 (2019) 957-980, incorporatedherein by reference in their entirety. Thus, CO₂ capture andsequestration is a key area of research.

A number of processes have been developed and utilized for CO₂ capture;the most common one is CO₂ absorption into an aqueous solution of asuitable amine(s). Among these, adsorption by amine solutions such asmonoethanolamine (MEA), diethanolamine (DEA), and methyldiethanolamine(MDEA) are most common. See P. J. G. Huttenhuis, N. J. Agrawal, J. A.Hogendoorn, G. F. Versteeg, Gas solubility of H₂S and CO₂ in aqueoussolutions of N-methyldiethanolamine, Journal of Petroleum Science andEngineering, 55 (2007) 122-134; H. Zare Aliabad, S. Mirzaei, Removal ofCO₂ and H₂S using Aqueous Alkanolamine Solutions, World Academy ofScience, Engineering and Technology, 3 (2009) 50-59; E. B. Rinker, S. S.Ashour, O. C. Sandall, Absorption of carbon dioxide into aqueous blendsof diethanolamine and methyldiethanolamine, Industrial & engineeringchemistry research, 39 (2000) 4346-4356; C. H. Yu, C. H. Huang, C. S.Tan, A Review of CO₂ Capture by Absorption and Adsorption, Aerosol andAir Quality Research, 12 (2012) 745-769; and A. A. Olajire, CO₂ captureand separation technologies for end-of-pipe applications energy 35(2010) 2610-2628, each incorporated herein by reference in theirentirety. However, this process is corrosive, energy-intensive, andutilizes environmentally unfriendly amines. Accordingly, severalalternatives such as cryogenic distillation, adsorption, membraneseparation, and biological processes (e.g., microbial treatment) havebeen sought. See K. Maqsood, A. Mullick, A. Ali, K. Kargupta, S.Ganguly, Cryogenic carbon dioxide separation from natural gas: A reviewbased on conventional and novel emerging technologies, Reviews inChemical Engineering, 30 (2014) 453-477; H. Bamdad, K. Hawboldt, S.MacQuarrie, A review on common adsorbents for acid gases removal: Focuson biochar, Renewable and Sustainable Energy Reviews, 81 (2018)1705-1720; F. D. Lamari, P. Langlois, M. Dicko, C. Chile, C. Chilev, I.Pentchev, Carbon dioxide capture by adsorption, Journal of ChemicalTechnology and Metallurgy, 51 (2016) 609-626; G. George, N. Bhoria, S.AlHallaq, A. Abdala, V. Mittal, Polymer membranes for acid gas removalfrom natural gas, Separation and Purification Technology, 158 (2016)333-356; B. Belaissaoui, E. Favre, Membrane Separation Processes forPost-Combustion Carbon Dioxide Capture: State of the Art and CriticalOverview, Oil Gas Sci. Technol.—Rev. IFP Energies nouvelles, 69 (2014)1005-1020; and S. Choi, J. H. Drese, C. W. Jones, Adsorbent Materialsfor Carbon Dioxide Capture from Large Anthropogenic Point Sources,ChemSusChem, 2 (2009) 796-854, each incorporated herein by reference intheir entirety. Among these, adsorption is most attractive due to itssimplicity, low energy requirements, and ease of adsorbentregeneration/reuse.

Despite the above attractive characteristics of adsorption, many knownadsorbents (e.g., amines) underperform in terms of CO₂ adsorptioncapacity, energy consumption for regeneration, corrosive properties, andmass losses during use, making them less appealing to industry. Researchhas thus focused on developing new solid adsorbents for CO₂ capture withhigher CO₂ adsorption capacity. Generally, most of the developedadsorbents are microporous/mesoporous silica, carbon-based adsorbents,metal oxides, and metal-organic frameworks (MOFs). See X. Liu, J. Li, L.Zhou, D. Huang, Y. Zhou, Adsorption of CO₂, CH₄ and N₂ on orderedmesoporous silica molecular sieve, Chemical Physics Letters, 415 (2005)198-201; S.-H. Liu, Y.-C. Lin, Y.-C. Chien, H.-R. Hyu, Adsorption of CO2from Flue Gas Streams by a Highly Efficient and Stable AminosilicaAdsorbent, Journal of the Air & Waste Management Association, 61 (2011)226-233; M. M. Maroto-Valer, Z. Tang, Y. Zhang, CO₂ capture by activatedand impregnated anthracites, Fuel Processing Technology, 86 (2005)1487-1502; M. C. Castrillon, K. O. Moura, C. A. Alves, M. Bastos-Neto,D. C. S. Azevedo, J. Hofmann, J. Möllmer, W.-D. Einicke, R. Glaser, CO₂and H₂S Removal from CH₄-Rich Streams by Adsorption on Activated CarbonsModified with K₂CO₃, NaOH, or Fe₂O₃, Energy & Fuels, 30 (2016)9596-9604; L. Li, X. Wen, X. Fu, F. Wang, N. Zhao, F. Xiao, W. Wei, Y.Sun, MgO/Al₂O₃ Sorbent for CO₂ Capture, Energy & Fuels, 24 (2010)5773-5780; A. Hakim, T. S. Marliza, N. M. Abu Tahari, R. W. N. WanIsahak, R. M. Yusop, W. M. Mohamed Hisham, A. M. Yarmo, Studies on CO₂Adsorption and Desorption Properties from Various Types of Iron Oxides(FeO, Fe₂O₃, and Fe₃O₄), Industrial & Engineering Chemistry Research, 55(2016) 7888-7897; T. Remy, S. A. Peter, S. Van der Perre, P. Valvekens,D. E. De Vos, G. V. Baron, J. F. M. Denayer, Selective Dynamic CO₂Separations on Mg-MOF-74 at Low Pressures: A Detailed Comparison with13X, The Journal of Physical Chemistry C, 117 (2013) 9301-9310; and S.Couck, J. F. M. Denayer, G. V. Baron, T. Rémy, J. Gascon, F. Kapteijn,An Amine-Functionalized MIL-53 Metal—Organic Framework with LargeSeparation Power for CO₂ and CH₄, Journal of the American ChemicalSociety, 131 (2009) 6326-6327, each incorporated herein by reference intheir entirety. Although these adsorbents show promising results, mostof them require high pressure or temperature to function effectively.See S. Choi, J. H. Drese, C. W. Jones, Adsorbent Materials for CarbonDioxide Capture from Large Anthropogenic Point Sources, ChemSusChem, 2(2009) 796-854, incorporated herein by reference in its entirety.Moreover, some of these adsorbents are non-regenerable or requireprohibitively high regeneration energy. See S. Kumar, S. K. Saxena, Acomparative study of CO₂ sorption properties for different oxides,Materials for Renewable and Sustainable Energy, 3 (2014) 1-30,incorporated herein by reference in its entirety.

For example, U.S. Pat. No. 5,087,597A discloses a method for the captureof CO₂ using a silica gel adsorbent with a maximum CO₂ uptake capacityat ambient conditions of 19.6 mg/g.

U.S. Pat. No. 5,876,488A discloses the synthesis of an amine supportedon AMBERLITE and its use for CO₂ adsorption, for example to reduce thepartial pressure of CO₂ in a gas phase mixture to about 1 mm Hg.

U.S. Pat. No. 5,492,683 discloses a method for preparing a CO₂ adsorbentcontaining 11 wt. % polyethyleneimine, 16 wt. % triethylene glycol, and73 wt. % AMBERLITE XAD-7, with a CO₂ adsorption capacity at 50° C. ofabout 40 mg/g.

U.S. Pat. No. 4,810,266A discloses a method for CO₂ adsorption usingaminated carbon molecular sieves that have been treated withalkanolamines, with the best adsorbent providing a maximum CO₂ uptakecapacity of below 70 mg/g.

U.S. Pat. No. 7,795,175B2 discloses a method for amine and polyaminegrafting on a fumed silica surface, with the adsorbents providing CO₂adsorption capacities of up to 105 mg/g.

U.S. Pat. No. 6,908,497B1 discloses a method for preparing dry amine-and/or alcohol-based composites of anhydrous calcium sulfate (CaSO₄),and other materials such as silica (SiO₂) and alumina (Al₂O₃) havingmaximum CO₂ adsorption capacities of roughly 35 mg/g at ambientconditions.

As alternatives to the above-mentioned adsorbents, metal oxides andmetal salts such as CaO, Li₄SiO₄, and MgO have also been used for CO₂adsorption. See A. M. Kierzkowska, R. Pacciani, C. R. Müller, CaO-BasedCO₂ Sorbents: From Fundamentals to the Development of New, HighlyEffective Materials, ChemSusChem, 6 (2013) 1130-1148; H. Lu, E. P.Reddy, P. G. Smirniotis, Calcium Oxide Based Sorbents for Capture ofCarbon Dioxide at High Temperatures, Industrial & Engineering ChemistryResearch, 45 (2006) 3944-3949; R. Quinn, R. J. Kitzhoffer, J. R. Hufton,T. C. Golden, A High Temperature Lithium Orthosilicate-Based SolidAbsorbent for Post Combustion CO₂ Capture, Industrial & EngineeringChemistry Research, 51 (2012) 9320-9327; and A. Hanif, S. Dasgupta, A.Nanoti, Facile Synthesis of High-Surface-Area Mesoporous MgO withExcellent High-Temperature CO₂ Adsorption Potential, Industrial &Engineering Chemistry Research, 55 (2016) 8070-8078, each incorporatedherein by reference in their entirety. Although these metallicadsorbents can function well at atmospheric pressure, they are typicallyonly effective at relatively high temperatures. High operatingtemperatures in some cases can cause a severe structural deformation ofthe adsorbents, deteriorating their adsorption capability upon extendeduse. See A. M. Kierzkowska, R. Pacciani, C. R. Müller, CaO-Based CO₂Sorbents: From Fundamentals to the Development of New, Highly EffectiveMaterials, ChemSusChem, 6 (2013) 1130-1148; and G. S. Grasa, J. C.Abanades, CO₂ Capture Capacity of CaO in Long Series ofCarbonation/Calcination Cycles, Industrial & Engineering ChemistryResearch, 45 (2006) 8846-8851, each incorporated herein by reference intheir entirety. In addition to the above limitations, metal oxideadsorbents require high regeneration temperatures, increasing energyconsumption and, thus, the increased operational costs for theadsorption process. See. Kumar, S. K. Saxena, A comparative study of CO₂sorption properties for different oxides, Materials for Renewable andSustainable Energy, 3 (2014) 1-30, incorporated herein by reference inits entirety. Moreover, the formation of metal carbides through thereaction between CO₂ and the metal oxides at high regenerationtemperatures makes regeneration a very challenging task. See H. Lu, E.P. Reddy, P. G. Smirniotis, Calcium Oxide Based Sorbents for Capture ofCarbon Dioxide at High Temperatures, Industrial & Engineering ChemistryResearch, 45 (2006) 3944-3949, incorporated herein by reference in itsentirety.

For example, U.S. Pat. No. 9,370,743B2 discloses a method for thesynthesis of a barium titanate composite adsorbent for CO₂ capture. Theadsorbent could not adsorb more than 50 mg/g CO₂ at ambient conditions,but increasing the adsorption temperature to 200-550° C. improved theCO₂ adsorption capability.

Magnesium oxide has been used in combination with metal salts as anadsorbent for CO₂ at high temperatures, and also for other purposes,such as remediation of toxic pollutants and as a catalyst support in anumber of reactions. See G. Xiao, R. Singh, A. Chaffee, P. Webley,Advanced adsorbents based on MgO and K₂CO₃ for capture of CO₂ atelevated temperatures, International Journal of Greenhouse Gas Control,5 (2011) 634-639; X. Zhao, G. Ji, W. Liu, X. He, E. J. Anthony, M. Zhao,Mesoporous MgO promoted with NaNO₃/NaNO₂ for rapid and high-capacity CO₂capture at moderate temperatures, Chemical Engineering Journal, 332(2018) 216-226; J. Hu, Z. Song, L. Chen, H. Yang, J. Li, R. Richards,Adsorption Properties of MgO(111) Nanoplates for the Dye Pollutants fromWastewater, Journal of Chemical & Engineering Data, 55 (2010) 3742-3748;J. H. Lunsford, P. Qiu, M. P. Rosynek, Z. Yu, Catalytic Conversion ofMethane and Ethylene to Propylene, The Journal of Physical Chemistry B,102 (1998) 167-173; and D. Szmigiel, W. Raróg-Pilecka, E. Miśkiewicz, M.Gliński, M. Kielak, Z. Kaszkur, Z. Kowalczyk, Ammonia synthesis overruthenium catalysts supported on high surface area MgO and MgO—Al₂O₃systems, Applied Catalysis A: General, 273 (2004) 105-112, eachincorporated herein by reference in their entirety.

However, there is still a need for new MgO-based adsorbents for CO₂capture technologies that are effective at ambient conditions.

SUMMARY OF THE INVENTION

Accordingly, it is one object of the present invention to provide anaminated magnesium oxide adsorbent, containing (i) a magnesium oxidematrix having disordered mesopores and a BET surface area of 320 to 380m²/g, and (ii) a polyamine selected from the group consisting of anethyleneamine having a molecular weight of up to 450 g/mol and apolyethylene imine having a number average molecular weight of greaterthan 500 g/mol and up to 20,000 g/mol, wherein the polyamine isimpregnated within the disordered mesopores of the magnesium oxidematrix.

In some embodiments, the magnesium oxide matrix is prepared fromprecipitation of magnesium hydroxide from a solution of a magnesium saltand ammonium hydroxide, followed by calcination of the magnesiumhydroxide at 350 to 450° C.

In some embodiments, the magnesium oxide matrix has an average porevolume of 0.3 to 0.5 cm³/g and an average pore size of 3 to 6 nm.

In some embodiments, the magnesium oxide matrix consists essentially ofmagnesium oxide.

In some embodiments, the polyamine is the ethyleneamine.

In some embodiments, the ethyleneamine is at least one selected from thegroup consisting of diethylenetriamine, triethylentetramine,tetraethylenepentamine, pentaethylene hexamine, and hexaethyleneheptamine.

In some embodiments, the ethyleneamine is diethylenetriamine.

In some embodiments, the polyamine is the polyethylene imine, and thepolyethylene imine is a linear polyethylene imine.

In some embodiments, the aminated magnesium oxide adsorbent has amagnesium content of 15 to 31 wt. %, an oxygen content of 40 to 50 wt.%, a carbon content of 17.5 to 30 wt. %, and a nitrogen content of 2 to16 wt. %, each based on a total weight of the aminated magnesium oxideadsorbent.

In some embodiments, the aminated magnesium oxide adsorbent has a BETsurface area of 40 to 98 m²/g.

In some embodiments, the aminated magnesium oxide adsorbent has anaverage pore size of 7 to 11 nm.

In some embodiments, the aminated magnesium oxide adsorbent has anaverage pore volume of 0.05 to 0.3 cm³/g.

In some embodiments, the aminated magnesium oxide adsorbent iscrystalline by XRD.

In some embodiments, the aminated magnesium oxide adsorbent has a CO₂uptake capacity of 24 to 60 mg CO₂ per 1 g of the aminated magnesiumoxide adsorbent at 30° C. and 1 atm.

It is another object of the present disclosure to provide a method ofmaking the aminated magnesium oxide adsorbent, the method involving (i)precipitating magnesium hydroxide from a solution of a magnesium saltand ammonium hydroxide, (ii) calcining the magnesium hydroxide at 350 to450° C. for 6 to 24 hours to form the magnesium oxide matrix havingdisordered mesopores, and (iii) wet impregnating the magnesium oxidematrix with an alcoholic solution of the polyamine.

In some embodiments, a molar ratio of the ammonium hydroxide to themagnesium salt in the solution is 2:1 to 9:1, and the precipitating isperformed by heating the solution to 50 to 70° C. for 3 to 10 hours,followed by stirring the solution at 20 to 30° C. for 12 to 48 hours.

In some embodiments, a weight ratio of the polyamine to the magnesiumoxide matrix for the wet impregnating is 1:1 to 1:3.

It is yet another object of the present disclosure to provide a methodof capturing CO₂ from a gas mixture containing CO₂ and at least oneother gas selected from the group consisting of hydrogen, oxygen,nitrogen, methane, and carbon monoxide, the method involving contactingthe gas mixture with the aminated magnesium oxide adsorbent to adsorb atleast a portion of the CO₂ into the aminated magnesium oxide adsorbent,thereby forming a loaded aminated magnesium oxide adsorbent and a gasstream depleted in CO₂ compared to the gas mixture.

In some embodiments, the gas mixture is a pre-combustion gas mixturecontaining 15 to 50 vol. % of CO₂, based on a total volume of the gasmixture.

In some embodiments, the gas mixture is a post-combustion gas mixturecontaining 5 to 15 vol. % of CO₂, based on a total volume of the gasmixture.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. A more complete appreciation of the invention and many of theattendant advantages thereof will be readily obtained as the samebecomes better understood by reference to the following detaileddescription when considered in connection with the accompanyingdrawings, wherein:

FIG. 1 shows the XRD patterns of the unmodified MgO-A (synthesized usingthe ammonium hydroxide route) and the MgO-A adsorbent functionalizedwith diethylenetriamine (DETA), referred to herein as “DETA-MgO-A”.

FIG. 2 shows the FTIR patterns of the unmodified MgO-A and theDETA-MgO-A adsorbent.

FIG. 3 shows the CO₂ capture at 30° C. and 1 atm using the unmodifiedMgO-A and the DETA-MgO-A adsorbent from a pure CO₂ gas stream flowing at100 mL/min.

FIG. 4 shows the adsorption-regeneration cycles of CO₂ on DETA-MgO-A,with adsorption taking place at 30° C. and 1 atm from a pure CO₂ gasstream flowing at 100 mL/min, and with regeneration being performed at120° C. and 1 atm with a pure N₂ gas stream flowing at 100 mL/min.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, it is understood that other embodimentsmay be utilized and structural and operational changes may be madewithout departure from the scope of the present embodiments disclosedherein.

Definitions

The phrase “substantially free”, unless otherwise specified, describes aparticular component being present in an amount of less than about 1 wt.%, preferably less than about 0.5 wt. %, more preferably less than about0.1 wt. %, even more preferably less than about 0.05 wt. %, yet evenmore preferably 0 wt. %, relative to a total weight of the compositionbeing discussed.

As used herein, the terms “optional” or “optionally” means that thesubsequently described event(s) can or cannot occur or the subsequentlydescribed component(s) may or may not be present (e.g., 0 wt. %).

The term “alkyl”, as used herein, unless otherwise specified, refers toa straight, branched, or cyclic, aliphatic (non-aromatic) fragmenthaving 1 to 26 carbon atoms, (e.g., C₁, C₂, C₃, C₄, C₅, C₆, C₇, C₈, C₉,C₁₀, C₁₁, C₁₂, C₁₃, C₁₄, etc.) and specifically includes, but is notlimited to, saturated alkyl groups such as methyl, ethyl, propyl,isopropyl, butyl, isobutyl, t-butyl, pentyl, isopentyl, neopentyl,hexyl, isohexyl, 3-methylpentyl, 2,2-dimethylbutyl, 2,3-dimethylbutyl,2-ethylhexyl, heptyl, octyl, nonyl, 3,7-dimethyloctyl, decyl, undecyl,dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl,octadecyl, nonadecyl, eicosyl, guerbet-type alkyl groups (e.g.,2-methylpentyl, 2-ethylhexyl, 2-proylheptyl, 2-butyloctyl,2-pentylnonyl, 2-hexyldecyl, 2-heptylundecyl, 2-octyldodecyl,2-nonyltridecyl, 2-decyltetradecyl, and 2-undecylpentadecyl);unsaturated alkenyl and alkynyl variants such as vinyl, allyl,1-propenyl, 2-propenyl, 1-butenyl, 2-butenyl, 3-butenyl, 1-pentenyl,2-pentenyl, 3-pentenyl, 4-pentenyl, 1-hexenyl, 2-hexenyl, 3-hexenyl,4-hexenyl, 5-hexenyl, oleyl, linoleyl; and cyclic alkyl groups(cycloalkyls) such as cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl,norbornyl, and adamantyl.

The term “aryl” means a carbocyclic aromatic monocyclic group containing6 carbon atoms which may be further fused to a second 5- or 6-memberedcarbocyclic group which may be aromatic, saturated or unsaturated. Arylincludes, but is not limited to, phenyl, anthracenyl, indanyl,1-naphthyl, 2-naphthyl, and tetrahydronaphthyl. The fused aryls may beconnected to another group either at a suitable position on thecycloalkyl/cycloalkenyl ring or the aromatic ring.

The term “arylalkyl”, as used herein, refers to a straight or branchedchain alkyl moiety (as defined above) that is substituted by an arylgroup (as defined above), examples of which include, but are not limitedto, benzyl, phenethyl, 2-methylbenzyl, 3-methylbenzyl, 4-methylbenzyl,2,4-dimethylbenzyl, 2-(4-ethylphenyl)ethyl, 3-(3-propylphenyl)propyl,and the like.

Throughout the specification and the appended claims, a given chemicalformula or name shall encompass all isomers (stereo and optical isomersand racemates) thereof where such isomers exist. Unless otherwiseindicated, all chiral (enantiomeric and diastereomeric) and racemicforms are within the scope of the disclosure. Many geometric isomers ofC═C double bonds, C═N double bonds, ring systems, and the like can alsobe present in the compounds, and all such stable isomers arecontemplated in the present disclosure. Cis- and trans- (or E- and Z-)geometric isomers of the compounds of the present disclosure aredescribed and may be isolated as a mixture of isomers or as separatedisomeric forms. The present compounds can be isolated in opticallyactive or racemic forms. Optically active forms may be prepared byresolution of racemic forms or by synthesis from optically activestarting materials. All processes used to prepare compounds of thepresent disclosure and intermediates made therein are considered to bepart of the present disclosure. When enantiomeric or diastereomericproducts are prepared, they may be separated by conventional methods,for example, by chromatography, fractional crystallization, or throughthe use of a chiral agent. Depending on the process conditions the endproducts of the present disclosure are obtained either in free (neutral)or salt form. Both the free form and the salts of these end products arewithin the scope of the disclosure. If so desired, one form of acompound may be converted into another form. A free base or acid may beconverted into a salt; a salt may be converted into the free compound oranother salt; a mixture of isomeric compounds of the present disclosuremay be separated into the individual isomers. Compounds of the presentdisclosure, free form and salts thereof, may exist in multipletautomeric forms, in which hydrogen atoms are transposed to other partsof the molecules and the chemical bonds between the atoms of themolecules are consequently rearranged. It should be understood that alltautomeric forms, insofar as they may exist, are included within thedisclosure. Further, a given chemical formula or name shall encompassall conformers, rotamers, or conformational isomers thereof where suchisomers exist. Different conformations can have different energies, canusually interconvert, and are very rarely isolatable. There are somemolecules that can be isolated in several conformations. For example,atropisomers are isomers resulting from hindered rotation about singlebonds where the steric strain barrier to rotation is high enough toallow for the isolation of the conformers. It should be understood thatall conformers, rotamers, or conformational isomer forms, insofar asthey may exist, are included within the present disclosure.

Aminated Magnesium Oxide Adsorbent

The present disclosure provides an aminated magnesium oxide adsorbentthat contains a desirable content of amino functional groups, and thathas suitable surface properties (e.g., BET surface area, pore volume,pore size etc.) for use as a selective adsorbent of CO₂ gas for CO₂capture technologies.

The aminated magnesium oxide adsorbent of the present disclosurecontains (i) a magnesium oxide matrix (mesoporous) and (ii) at least onepolyamine which is impregnated within the pores of the magnesium oxidematrix. Impregnation with the polyamine provides the aminated magnesiumoxide adsorbent with amino functional groups which help facilitate theselective adsorption of CO₂.

(i) Magnesium Oxide Matrix

The aminated magnesium oxide adsorbent is made from a porous matrix ofmagnesium oxide. In preferred embodiments, the matrix is composed ofsubstantially pure magnesium oxide, with an elemental composition ofmagnesium of 45 to 60 wt. %, preferably 45.5 to 58 wt. %, preferably 46to 56 wt. %, preferably 46.5 to 54 wt. %, preferably 47 to 52 wt. %,preferably 47.2 to 50 wt. %, and oxygen of 40 to 55 wt. %, preferably 42to 54.5 wt. %, preferably 44 to 54 wt. %, preferably 46 to 53.5 wt. %,preferably 48 to 53 wt. %, preferably 50 to 52.8 wt. %. While thecomposition of the matrix may vary depending on a number of factors,such as the purity of reagents, the type of reactants/conditionsemployed for the synthesis of the magnesium oxide, etc., it is preferredthat the magnesium oxide matrix employed herein consists essentially ofmagnesium oxide. In this context, “consists essentially of” is intendedto mean a magnesium oxide matrix made of only magnesium and oxygenelements, and optionally residual water (e.g., water molecules entrappedwithin the pores of the magnesium oxide matrix, moisture derived fromstandard atmosphere, etc.) and/or residual surface hydroxyl groups, andwhich contains no other elements such as carbon, nitrogen, silicon,aluminum, potassium, sodium, calcium, transition metals (e.g.,zirconium, titanium, iron, copper, chromium, cerium, niobium, vanadium,yttrium, barium, etc.), etc. Thus, “consists essentially of” is intendedto exclude from the matrix impurities or other materials which wouldcontribute such above elements including, but not limited to,

-   -   carbon/organic impurities/materials such as oxalic acid,        graphene, graphene oxide, activated carbon, fibers or comminuted        materials sourced from plants or agricultural products such as        fibers or comminuted materials sourced from from the husks,        shells, stems, roots, leaves (or fronds or leaflets), cores,        trunks, inflorescences, fruit, pulp, empty fruit bunches, seeds        (pit), or the offshoots of various other plants or agricultural        products (e.g., the seeds/nuts and/or seed/nut shells or hulls        of almond, brazil, cocoa bean, coconut, cotton, flax, grass,        linseed, maize, millet, oat, peach, apricot, date pit/date        stones, peanut, rye, soybean, sunflower, walnut, wheat; rice        straw; rice bran; rice husk including rice husk ash; crude        pectate pulp; peat moss fibers; flax; cotton; cotton linters;        wool; sugar cane; jute stick; neem leaves; paper; bagasse;        bamboo; corn stalks; wood/wood chips/wood fibers/wood pulp;        bark; straw such as wheat straw; pine cone; cork; dehydrated        vegetable matter; whole ground corn cobs; corn stalks; corn cob        light density pith core; corn cob ground woody ring portion;        corn cob chaff portion; cotton seed stems; flax stems; wheat        stems; sunflower seed stems; soybean stems; maize stems; rye        grass stems; millet stems; cellulosic fibers; cellulose; coconut        palm materials such as coconut shells, coconut husks; and oil        palm materials such as palm oil fuel ash, palm oil fibers, palm        oil shells, and palm oil empty fruit brunches).    -   metal oxides such as calcium oxide, potassium oxide, iron oxide,        titanium oxide, silica (SiO₂), alumina (Al₂O₃), alumina        tri-hydroxides, aluminum oxide hydroxides (e.g., boehmite),        zirconia, silicates such as aluminosilicate (e.g.,        silica-alumina, zeolites) and magnesium iron aluminum        cyclosilicates (e.g., cordierite), ceria, niobia, vanadia,        magnesium stabilized zirconia, zirconia stabilized alumina,        yttrium stabilized zirconia, calcium stabilized zirconia,        magnesium stabilized alumina, calcium stabilized alumina;    -   metals such as sodium, zirconium, titanium, iron, copper,        chromium, lanthanum, cerium, barium, aluminum titanate, silicon        nitride, silicon carbide, including metal ions which may be part        of a metal-organic framework (MOF) structure;    -   among others.

While magnesium oxide may be produced by various methods (e.g.,precipitation with sodium hydroxide or oxalic acid, followed bycalcination etc.), the inventors have found that an advantageousmagnesium oxide material, in terms of surface properties for CO₂adsorption, is produced by precipitation of magnesium hydroxide from asolution of a magnesium salt and ammonium hydroxide, followed bycalcination of the magnesium hydroxide at 350 to 450° C., as will bediscussed in more detail later. Such a method produces a magnesium oxidematrix formed from individual nanoparticles of MgO having an irregularplatelet-like morphology, for example irregular MgO nanoplatelets in thesize range of 2 to 100 nm, preferably 5 to 90 nm, preferably 10 to 80nm, preferably 15 to 70 nm, preferably 20 to 60 nm, preferably 25 to 50nm, preferably 30 to 45 nm, preferably 35 to 40 nm.

The manner in which the magnesium oxide matrix is made also effects thesurface structure/properties of the resulting magnesium oxide matrix,and thus the absorbency properties of the eventual aminated magnesiumoxide adsorbent of the present disclosure. As mentioned previously, themagnesium oxide matrix is preferably prepared from precipitation ofmagnesium hydroxide through the use of ammonium hydroxide, followed bycalcination at 350 to 450° C., and the resulting magnesium oxide matrixcontains disordered mesopores. The disordered pore structure of themagnesium oxide matrix thus differs from other materials having anordered pore structure/network such as honeycomb or monolithic materialsincluding flow-through monoliths, wall-flow monoliths, or partial-flowmonoliths (e.g., honeycomb or monolithic MgO), molecular organicframeworks (MOFs), zeolitic materials containing ordered mesoporousnetworks (e.g., MCM-41, SBA-15, ZSM-5, etc.), and the like.

In terms of surface structure/properties, the preferred magnesium oxidematrix of the present disclosure (formed using ammonium hydroxide) hasand an average pore size of 3 to 6 nm, preferably 3.4 to 5.8 nm,preferably 3.8 to 5.4 nm, preferably 4.2 to 5.2 nm, preferably 4.6 to5.0 nm, preferably 4.7 to 4.8 nm. In preferred embodiments, themagnesium oxide material contains only mesopores, and no micropores ormacropores are present.

In some embodiments, magnesium oxide matrix has a BET surface area of320 to 380 m²/g, preferably 330 to 370 m²/g, preferably 335 to 365 m²/g,preferably 340 to 360 m²/g, preferably 345 to 355 m²/g, preferably 350to 351 m²/g.

In some embodiments, the magnesium oxide matrix has an average porevolume of 0.3 to 0.5 cm³/g, preferably 0.32 to 0.48 cm³/g, preferably0.34 to 0.46 cm³/g, preferably 0.36 to 0.44 cm³/g, preferably 0.38 to0.43 cm³/g, preferably 0.4 to 0.42 cm³/g.

The magnesium oxide matrix is preferably crystalline by X-raydiffraction (XRD), preferably having a cubic MgO crystal structure withdiffraction peaks at 36.8°, 42.2°, 61.2°, 74.1° and 77.9°, correspondingto the planes (111), (200), (220), (311), and (222) of cubic MgO,respectively.

(ii) Polyamine

The aminated magnesium oxide adsorbent also contains a polyamine, whichis a chemical compound that contains at least two amino groups permolecule (e.g., 2, 3, 4 or more amino groups), preferably at least 3amino groups per molecule. The amino groups may be primary amino groups(H₂N—R), secondary amino groups (R—N(H)—R), tertiary amino groups(R—N(R)—R), or a mixture thereof. In preferred embodiments, the aminogroups are a mixture of primary and secondary amino groups.

Impregnation (intercalation) of the magnesium oxide matrix with thepolyamine preferably results in the polyamine being drawn into the porespaces (disordered mesopores) of the magnesium oxide matrix throughcapillary action. In some embodiments, the polyamine resides within thedisordered mesopores of the magnesium oxide matrix, but is notcovalently attached/bonded to the magnesium oxide matrix. In someembodiments, the polyamine resides within the disordered mesopores ofthe magnesium oxide matrix, and at least one amino group of thepolyamine is datively bonded (coordinate covalent bond) to the magnesiumoxide matrix (e.g., N→Mg), or otherwise associated with the magnesiumoxide matrix through other forces such as Van der Waals forces. In suchcases, it is preferred that not all of the amino groups present in thepolyamine are covalently coordinated to the magnesium oxide matrix, andat least one of the amino functional groups of the polyamine remainunbound and capable of interacting with carbon dioxide for carbondioxide capture applications. In addition to being located within thedisordered mesopores of the magnesium oxide matrix, the polyamine mayalso be datively bound to (or otherwise bound to) an external surface ofthe magnesium oxide matrix particles. External surfaces are thoseaccessible surfaces which are located at or above the basal plane of theaminated magnesium oxide adsorbent material (not within a pore). On theother hand, surfaces within the pore spaces of the aminated magnesiumoxide adsorbent are regarded as internal surfaces since they lie belowthe basal plane of the aminated magnesium oxide adsorbent material. Inother words, even though a pore wall may be considered an ‘external’surface since it is connected to and accessible from the outsideenvironment, because the pore wall is located beneath the basal plane itis considered to be in an internal surface herein.

The polyamine may be an ethyleneamine (small molecule or oligomericcompound) or a polyethylene imine (polymeric compound), which aredistinguishable herein at least in terms of their molecular weight.

<ethyleneamine> In some embodiments, the polyamine is an ethyleneamine,which is a class of small molecule or oligomeric compounds containing atleast one ethylene (—CH₂CH₂—) linkage between amino groups. Theethyleneamine of the present disclosure has a molecular weight of up to450 g/mol. For example, the ethylenamine may have a molecular weight of60 to 450 g/mol, preferably 65 to 350 g/mol, preferably 70 to 300 g/mol,preferably 75 to 275 g/mol, preferably 80 to 250 g/mol, preferably 85 to225 g/mol, preferably 90 to 200 g/mol, preferably 95 to 175 g/mol,preferably 100 to 150 g/mol.

In some embodiments, the ethyleneamine has an amine value of 1,000 to1855 mg KOH/g, preferably 1,100 to 1,855 mg KOH/g, preferably 1,200 to1,800 mg KOH/g, preferably 1,300 to 1,750 mg KOH/g, preferably 1,400 to1,700 mg KOH/g, preferably 1,500 to 1,650 mg KOH/g, preferably 1,600 to1,630 mg KOH/g.

The ethyleneamine may be a linear compound containing only primaryand/or secondary amino groups, a branched compound containing at leastone tertiary amino group (and one or more of a primary and/or secondaryamino group), or a cyclic compound containing at least one nitrogenousheterocyclic structure in addition to an ethylene linkage. In someembodiments, the ethyleneamine is an acyclic compound. In preferredembodiments, the ethylenamine is a linear compound, most preferably alinear compound containing primary and secondary amino groups.

Exemplary ethyleneamines include, but are not limited to, ethylenediamine, aminoethylethanolarnine (AEEA), 1,2-diaminopropane, 12-diaminocyclohexane, 2,3-diaminobutane, 2,3-diaminobutan-1-ol,propane-1,2,3-triamine, tris(2-aminoethyl)amine, aminoethylpiperazine,N,N′-bis-(2-aminoethyl)piperazine),N-[(2-aminoethyl)-2-aminoethyl]piperazine, diethylenetriamine (DETA),triethylentetramine (TETA), tetraethylenepentamine (TEPA), pentaethylenehexamine (PEHA), hexaethylene heptamine (HEHA), aminoethyltriethylentetramine (AETETA), as well as mixtures of ethyleneamines suchas ETHYLENEAMINE E-100 (a mixture of TEPA, PEHA, HEHA and highermolecular weight products with a number average molecular weight of250-300 g/mol, available from Huntsman Corporation) and HEAVY POLYAMINEX (HPA-X) (a mixture of linear, branched, and cyclic ethyleneaminescomprising TETA, TEPA, PEHA, and higher molecular weight products with aweight average molecular weight of 275 g/mol from Dow Chemical Company.

In preferred embodiments, the ethyleneamine is a linear compound offormula (I):

wherein n is a positive integer from 1 to 10, preferably from 2 to 6,preferably from 3 to 4. In preferred embodiments, the ethyleneamine isat least one selected from the group consisting of diethylenetriamine,triethylentetramine, tetraethylenepentamine, pentaethylene hexamine, andhexaethylene heptamine, more preferably diethylenetriamine (DETA).

<polyethylene imine> In some embodiments, the polyamine is apolyethylene imine (PEI), which is a polymer with a repeating unitcomposed of an amino group and an ethylene (—CH₂CH₂—) spacer. The PEI ofthe present disclosure has a number average molecular weight of greaterthan 500 g/mol and up to 20,000 g/mol, preferably 600 to 18,000 g/mol,preferably 800 to 16,000 g/mol, preferably 1,000 to 14,000 g/mol,preferably 2,000 to 12,000 g/mol, preferably 4,000 to 11,000 g/mol,preferably 6,000 to 10,000 g/mol, for example as determined by gelpermeation chromatography (GPC).

In some embodiments, the polyethylene imine has a polydispersity index(PDI) (Mw/Mn) of less than or equal to 2, preferably less than or equalto 1.8, preferably less than or equal to 1.6, preferably less than orequal to 1.4, preferably less than or equal to 1.3, preferably less thanor equal to 1.2, preferably less than or equal to 1.1.

The polyethylene imine may be a linear PEI (composed of mostly secondaryamino groups and primary amino groups), or a branched PEI (containsprimary, secondary and tertiary amino groups), as determined by thedegree of branching (DB). The degree of branching may be determined by¹³C-NMR spectroscopy, preferably in D₂O, and is defined as:

${DB} = \frac{\left( {D + T} \right)}{\left( {D + T + L} \right)}$wherein D (dendritic) corresponds to the fraction of tertiary aminogroups, L (linear) corresponds to the fraction of secondary amino groupsand T (terminal) corresponds to the fraction of primary amino groups. APEI with a degree of branching (DB) of <0.1 is considered herein to be alinear polymer, while a PEI with a DB of ≥0.1 to 1.0 is consideredherein to be a branched polymer.

In some embodiments, the polyethylene imine is a linear PEI, and has adegree of branching (DB) of less than 0.1, preferably less than 0.08,preferably less than 0.06, preferably less than 0.04, preferably lessthan 0.02, preferably less than 0.01. The linear PEI may be derived fromacidic hydrolysis of polyoxazoline, for example. In preferredembodiments, the polyethylene imine is a linear polyethylene imine.

In some embodiments, the polyethylene imine is a branched PEI, and has adegree of branching (DB) of 0.1 to 0.95, preferably 0.2 to 0.90,preferably 0.30 to 0.80, preferably 0.4 to 0.7, preferably 0.5 to 0.6.

In some embodiments, the aminated magnesium oxide adsorbent consists ofthe magnesium oxide matrix and the polyamine. In some embodiments, aweight ratio of the polyamine to the magnesium oxide matrix in theaminated magnesium oxide adsorbent ranges from 1:1 to 1:3, preferably1:1.2 to 1:2.8, preferably 1:1.4 to 1:2.6, preferably 1:1.6 to 1:2.4,preferably 1:1.8 to 1:2.2, preferably 1:2.

In preferred embodiments, the aminated magnesium oxide adsorbent isformed only from impregnation of the (i) the magnesium oxide matrix with(ii) the polyamine, and no other functionalization/impregnation agentsare introduced. Such other functionalization/impregnation agents mayinclude, but are not limited to,

-   -   alkyl silanes such as methyltrimethoxysilane,        methyltriethoxysilane, dimethyldiethoxysilane,        ethyltrimethoxysilane, trimethylethoxysilane,        dimethyldimethoxysilane, propyltriethoxysilane,        propyltrimethoxysilane, isobutyltriethoxysilane,        hexyltrimethoxysilane, hexyltriethoxysilane,        cyclohexyltriethoxysilane, cyclohexyltributoxysilane,        heptyltrimethoxysilane, heptyltriethoxysilane,        octyltrimethoxysilane, octyltriethoxysilane,        methyloctyldimethoxysilane, methyloctyldiethoxysilane,        nonyltrimethoxysilane, nonyltriethoxysilane,        decyltrimethoxysilane, decyltriethoxysilane,        dodecyltrimethoxysilane, dodecyltriethoxysilane,        tetradecyltrimethoxysilane, tetradecyltriethoxysilane,        octadecyltrimethoxysilane, and octadecyltriethoxysilane;    -   aryl silanes such as ethoxy(diphenyl)vinyl silane,        trichloro[4-(chloromethyl)phenyl] silane, dimethoxy(diphenyl)        silane, diethoxy(diphenyl) silane, diethoxy(methyl)phenyl        silane, trichloro(phenyl) silane, triethoxy(phenyl) silane, and        trimethoxy(phenyl) silane;    -   arylalkylsilanes such as trimethoxy(2-phenylethyl) silane;    -   orthosilicates such as tetraethyl orthosilicate (TEOS),        tetramethyl orthosilicate, tetrapropyl orthosilicate, tetrabutyl        orthosilicate, tetraallyl orthosilicate, tetrakis(dimethylsilyl)        orthosilicate, and tetraamyl orthosilicate;    -   halo- or glycidyl-containing silanes such as        (3-glycidyloxypropyl) trimethoxysilane (GTPMS),        [2-(3,4-epoxycyclohexyl)ethyl]trimethoxysilane, and        chloropropyltrimethoxysilane;    -   fluoroalkylsilanes such as        1,1,2,2-perfluorooctyl-trichlorosilane (PFOTS),        (heptadecafluoro-1,1,2-2-tetrahydrodecyl)triethoxysilane,        (3,3,3-trifluoropropyl)trimethoxysilane,        heptadecafluoro-1,1,2,2-tetrahydrodecyl)trichlorosilane,        chlorodimethyl(pentafluorophenyl)silane,        chloromethyl)methylbis(pentafluorophenyl)silane,        diisopropyl(3,3,4,4,5,5,6,6,6-nonafluorohexyl)silane,        diisopropyl(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane,        trichloro(3,3,3-trifluoropropyl)silane,        trichloro(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)silane,        triethoxy(4-(trifluoromethyl)phenyl)silane, and        tridecafluoro-1,1,2,2-tetrahydrooctyl)trimethoxysilane; and    -   amino silanes such as        N-(3-(trimethoxysilyl)propyl)ethane-1,2-diamine (AEAPTMS),        trimethoxysilylpropyldiethylenetriamine, trimethoxysilylpropyl        polyethyleneimine, 3-aminopropyl triethoxysilane (APTES),        3-aminopropyl trimethoxysilane (APTMS),        2-aminoethyltrimethoxysilane, 4-aminobutyltriethoxysilane,        5-aminopentyltrimethoxysilane, 6-aminohexylltrimethoxysilane,        7-aminoheptyltriethoxysilane,        bis-(2-hydroxyethyl)-3-aminopropyltriethoxysilane, with specific        mention being made to 3-aminopropyl triethoxysilane (APTES); and    -   monoamines such as methylamine, ethylamine, propylamine,        monoethanolamine (MEA), diethanolamine (DEA),        methyldiethanolamine (MDEA), as well as polyoxyalkylene amines        (e.g., ethoxylated monoamines) containing a single amino group.

Such other functionalization/impregnation agents may also include, butare not limited to, polymeric functionalization materials such aspolymeric polyamines other than polyethylene imines (e.g.,polyazetidines), polyamides, epoxy resins (e.g., epoxy chloropropane),peptides, and polyalkylene glycols for example polyethylene glycol.

While the use of intermediate coating/binding/functionalization layersare contemplated, in general the magnesium oxide matrix of the presentdisclosure is directly contacted with the polyamine, and no intermediatecoating layers, binding layers, or functionalization layers are present.For example, the magnesium oxide matrix is typically not pre-coated orotherwise pre-treated with a silica coating, a magnesium silicatecoating, etc. such as those precipitated silica coatings described in US2018/0326395A1—incorporated herein by reference in its entirety, or analumina sol, an aluminum oxide hydroxide, a silica sol, a titania sol, azirconium acetate, or silicone binders described in U.S. Pat. No.9,457,340B2—incorporated herein by reference in its entirety, or anyother binding layer (e.g., epoxy chloropropane,chloropropyltrimethoxysilane), prior to impregnation with the polyamine.

As a result of impregnating (i) the magnesium oxide matrix with (ii) thepolyamine, the aminated magnesium oxide adsorbent is produced. Theelemental constitution of the aminated magnesium oxide adsorbent mayvary depending on a number of factors, such as the purity of themagnesium oxide matrix employed in the impregnation process, polyamineemployed in the impregnation process, the relative ratio of the (i) and(ii) (the extent of amino functional group installation), etc. However,the aminated magnesium oxide adsorbent effective in the methods hereingenerally comprises, each based on a total weight of the aminatedmagnesium oxide adsorbent:

-   -   magnesium in an amount of 15 to 31 wt. %, preferably 16 to 30        wt. %, preferably 17 to 29.5 wt. %, preferably 17.3 to 29 wt. %,        preferably 18 to 28 wt. %, preferably 19 to 27 wt. %, preferably        20 to 26 wt. %, preferably 21 to 25 wt. %, preferably 22 to 24        wt. %,    -   oxygen in an amount of 40 to 50 wt. %, preferably 40.5 to 49 wt.        %, preferably 41 to 48 wt. %, preferably 41.5 to 47 wt. %,        preferably 42 to 46 wt. %, preferably 42.5 to 45 wt. %,        preferably 43 to 44 wt. %,    -   carbon in an amount of 17.5 to 30 wt. %, preferably 18 to 29 wt.        %, preferably 19 to 28 wt. %, preferably 20 to 27 wt. %,        preferably 21 to 26 wt. %, preferably 22 to 25 wt. %, preferably        23 to 24 wt. %, and    -   nitrogen in an amount of 1 to 6 wt. %, preferably 1.2 to 5.5 wt.        %, preferably 1.4 to 5 wt. %, preferably 1.6 to 4.5 wt. %,        preferably 1.8 to 4 wt. %, preferably 2 to 3.5 wt. %, preferably        2.2 to 3 wt. %, preferably 2.4 to 2.6 wt. %.

For example, in some embodiments, the aminated magnesium oxide adsorbentis formed from (i) the magnesium oxide matrix and (ii) an ethyleneamine(e.g., DETA), and comprises, each based on a total weight of theaminated magnesium oxide adsorbent: magnesium in an amount of 27 to 31wt. %, preferably 27.5 to 30.5 wt. %, preferably 28 to 30 wt. %,preferably 28.5 to 29.8 wt. %, preferably 29 to 29.6 wt. %, preferably29.5 wt. %; oxygen in an amount of 43 to 50 wt. %, preferably 44 to 49wt. %, preferably 45 to 48 wt. %, preferably 46 to 47 wt. %, preferably46.2 wt. %; carbon in an amount of 17.5 to 25 wt. %, preferably 18 to 24wt. %, preferably 19 to 23 wt. %, preferably 20 to 22 wt. %, preferably20.2 to 21 wt. %; and nitrogen in an amount of 2 to 5 wt. %, preferably2.5 to 4.8 wt. %, preferably 3 to 4.6 wt. %, preferably 3.5 to 4.4 wt.%, preferably 4 to 4.1 wt. %.

In preferred embodiments, the aminated magnesium oxide adsorbent issubstantially free of, preferably completely free of (0 wt. %), silicon,aluminum, potassium, sodium, calcium, and transition metals (e.g.,zirconium, titanium, iron, copper, chromium, cerium, niobium, vanadium,yttrium, barium, etc.).

Like the magnesium oxide prior to impregnation, the aminated magnesiumoxide adsorbent is preferably crystalline by X-ray diffraction (XRD).However, the aminated magnesium oxide adsorbent has some differentdiffraction peaks, with diffraction peaks at 36.8°, 42.2° (minor),50.2°, 58.7°, 61.2° (minor), and 72.2°.

Owing at least in part to impregnation/intercalation with the polyamine,which without being bound by theory may act to partially obstruct thepores of the magnesium oxide matrix, the aminated magnesium oxideadsorbent herein may have a reduced BET surface area and pore volume,and in some cases a drastically reduced BET surface area and porevolume, compared to the magnesium oxide matrix prior toimpregnation/intercalation. In some embodiments, the aminated magnesiumoxide adsorbent has a BET surface area of 40 to 98 m²/g, preferably 50to 97 m²/g, preferably 60 to 96 m²/g, preferably 70 to 95 m²/g,preferably 80 to 94 m²/g, preferably 85 to 93 m²/g, preferably 90 to 92m²/g, preferably 91 to 91.5 m²/g. In some embodiments, for example whenthe aminated magnesium oxide adsorbent is formed with a polyethyleneimine of high number average molecular weight (e.g., 5,000 to 20,000g/mol, preferably 10,000 to 15,000 g/mol), the aminated magnesium oxideadsorbent has a BET surface area of 3 to 35 m²/g, preferably 3.5 to 25m²/g, preferably 4 to 15 m²/g, preferably 4.3 to 5 m²/g.

In some embodiments, the aminated magnesium oxide adsorbent has anaverage pore volume of 0.05 to 0.3 cm³/g, preferably 0.1 to 0.28 cm³/g,preferably 0.14 to 0.26 cm³/g, preferably 0.16 to 0.24 cm³/g, preferably0.18 to 0.22 cm³/g, preferably 0.2 to 0.21 cm³/g. In some embodiments,for example when the aminated magnesium oxide adsorbent is formed with apolyethylene imine of high number average molecular weight (e.g., 5,000to 20,000 g/mol, preferably 10,000 to 15,000 g/mol), the aminatedmagnesium oxide adsorbent has an average pore volume of 0.001 to 0.1cm³/g, preferably 0.005 to 0.08 cm³/g, preferably 0.01 to 0.05 cm³/g

In some embodiments, the aminated magnesium oxide adsorbent has anaverage pore size of 7 to 11 nm, preferably 7.4 to 10.5 nm, preferably7.6 to 10 nm, preferably 8 to 9.5 nm, preferably 8.5 to 9 nm, preferably8.8 to 8.9 nm.

The aminated magnesium oxide adsorbent of the present disclosure has aneffective adsorption capacity for CO₂, which can be determined bythermodynamic, low pressure, single component gas adsorption isotherms.In some embodiments, the aminated magnesium oxide adsorbent has a CO₂uptake capacity, in terms of mg CO₂ per 1 g of the aminated magnesiumoxide adsorbent at 30° C. and 1 atm, of 24 to 60 mg/g, preferably 28 to58 mg/g, preferably 32 to 56 mg/g, preferably 36 to 54 mg/g, preferably40 to 52 mg/g, preferably 42 to 50 mg/g, preferably 44 to 49 mg/g,preferably 46 to 48.5 mg/g, preferably 48 mg/g. In some embodiments, forexample when the aminated magnesium oxide adsorbent is formed with apolyethylene imine of high number average molecular weight (e.g., 5,000to 20,000 g/mol, preferably 10,000 to 15,000 g/mol), the aminatedmagnesium oxide adsorbent has a CO₂ uptake capacity, in terms of mg CO₂per 1 g of the aminated magnesium oxide adsorbent at 30° C. and 1 atm,of 18 to 30 mg/g, preferably 20 to 28 mg/g, preferably 22 to 24 mg/g.

While combinations of the aminated magnesium oxide adsorbent of thepresent disclosure with other sieving/support materials is contemplated,preferably the adsorbent of the present disclosure is made from only (i)the magnesium oxide matrix and (ii) the polyamine, and no othersieving/support materials are present. Exemplary other sieving/supportmaterials include, but are not limited to, the impurities or othermaterials listed previously with respect to the matrix, with specificmention being made to hierarchical mesoporous silicates andaluminosilicates (zeolites) such as MCM-41, dendritic silica mesoporous,SBA-15, and ZSM-5; carbonaceous materials such as graphene, grapheneoxide, activated carbon; and molecular organic frameworks (MOFs) such asHKUST-1, ZIFs (e.g., ZIF-90), and UiO-66.

Methods of Making the Aminated Magnesium Oxide Adsorbent

The present disclosure also provides methods for making the aminatedmagnesium oxide adsorbent.

Briefly, such methods may first involve precipitation of magnesiumhydroxide from a solution of a magnesium salt and a precipitation agent.The precipitation may be performed by first dissolving a magnesium saltin water (e.g., distilled water), for example at a ratio (w/w) of 0.02:1to 0.15:1, preferably 0.04:1 to 0.10:1, preferably 0.06:1 to 0.09:1,preferably 0.07:1 to 0.08:1. Acceptable magnesium salts may include, butare not limited to, magnesium nitrate, magnesium chloride, ammoniummagnesium phosphate, magnesium bromide, magnesium carbonate, magnesiumperchlorate, magnesium phosphate, magnesium sulfate, as well as hydratesthereof. In preferred embodiments, the magnesium salt is magnesiumnitrate or a hydrate thereof (e.g., magnesium nitrate hexahydrate).

Next, a precipitation agent is added to effect precipitation ofmagnesium hydroxide from the resulting solution. The precipitation agentmay be employed at a molar ratio of the precipitation agent to themagnesium salt in the solution of from 2:1 to 9:1, preferably 3:1 to8:1, preferably 4:1 to 7:1, preferably 5:1 to 6:1. While the use ofvarious precipitation agents are contemplated herein, such as ammoniumhydroxide, oxalic acid, and sodium hydroxide, it is preferred thatammonium hydroxide is employed as the precipitation agent, as ammoniumhydroxide has been found to achieve advantageous results in terms of thestructure, surface properties, and adsorption capacity of the finalaminated magnesium oxide adsorbent formed, compared to when otherprecipitation agents are utilized such as oxalic acid and sodiumhydroxide. In preferred embodiments, the ammonium hydroxide is added tothe solution as a 10 to 36% solution in water (w/w), preferably 15 to30%, preferably 20 to 25%.

Upon addition of the precipitating agent, the solution may be firstagitated (e.g., stirred, shaken, sonicated, etc.) at room temperature(e.g., 20 to 25° C.), then heated to 50 to 70° C., preferably 52 to 68°C., preferably 54 to 66° C., preferably 56 to 64° C., preferably 58 to62° C., for 3 to 10 hours, preferably 4 to 9 hours, preferably 5 to 8hours, preferably 6 to 7 hours. At the end of the heating time period,the solution may be cooled to 20 to 30° C., preferably 20 to 25° C., andagitated (e.g., stirred) for an additional for 12 to 48 hours,preferably 16 to 36 hours, preferably 20 to 30 hours, preferably 24 to26 hours, thereby forming a precipitate.

The precipitate, which contains the magnesium hydroxide, may becollected using any known solid-liquid separation technique, forexample, filtration, decantation, centrifugation, etc., and washed withwater to remove any water-soluble byproduct (e.g., NH₄NO₃) as well asany unreacted metal salt that may remain. The washing may be performedas many times as needed to provide the magnesium hydroxide in desirablepurity. Prior to calcination, the magnesium hydroxide may be optionallydried in an oven at 50 to 80° C., preferably 55 to 75° C., preferably 60to 70° C., for 1 to 24 hours, preferably 5 to 20 hours, preferably 10 to16 hours, preferably 12 to 14 hours, and optionally ground.

Next, the magnesium hydroxide is calcined to form the magnesium oxidematrix. The calcination is preferably performed under air at 350 to 450°C., preferably 360 to 440° C., preferably 370 to 430° C., preferably 380to 420° C., preferably 390 to 410° C., preferably 400° C., for 6 to 24hours, preferably 7 to 20 hours, preferably 8 to 16 hours, preferably 9to 14 hours, preferably 10 to 12 hours to form the magnesium oxidematrix having disordered mesopores. While calcinations performed outsideof this range may be acceptable in some circumstances, it has been foundthat the above calcination temperature provides the magnesium oxidematrix with the most advantageous characteristics (e.g., crystallinity,morphology, porosity, composition, and adsorbent capacity). For example,calcinations performed at temperatures below this range may result inincomplete conversion of the magnesium hydroxide to magnesium oxide,which may reduce the adsorption capacity for CO₂. On the other hand,calcinations performed at temperatures above this range may cause anunacceptable level of MgO particle agglomeration, which may affect theporosity of the magnesium oxide matrix produced, thus significantlyreducing the CO₂ adsorption capacity of the material.

The obtained magnesium oxide matrix is then subject to impregnation withthe polyamine (described earlier), preferably the magnesium oxide matrixis wet impregnated with an alcoholic solution of the polyamine. Becauseimpregnation/intercalation with the polyamine results in theintroduction of amino functional groups within the pore spaces andoptionally the surface of the magnesium oxide matrix, the productresulting from such polyamine impregnation is referred to as “aminated”,i.e., the aminated magnesium oxide adsorbent.

Impregnation may be performed by first forming an alcoholic solution ofthe polyamine. Various concentrations of alcoholic solutions may beprepared, for example, the alcoholic solution may be prepared from 0.005to 0.1 g of polyamine per 1 mL of alcohol, preferably 0.008 to 0.05 g ofpolyamine per 1 mL of alcohol, preferably 0.01 to 0.02 g of polyamineper 1 mL of alcohol. The alcoholic solution may be optionally stirredand/or heated for any amount of time suitable for complete dissolutionof the polyamine in the alcohol. Suitable alcohols which may be utilizedinclude, but are not limited to, methanol, ethanol, propanol,isopropanol, n-butanol, isobutanol, n-pentanol, n-hexanol,3-methyl-3-buten-1-ol, ethylene glycol, diethylene glycol, triethyleneglycol, tetraethylene glycol, propylene glycol, dipropylene glycol,1,3-propanediol. In preferred embodiments, the alcohol is a monoalcohol,preferably ethanol.

Next, the magnesium oxide matrix obtained from above may be mixed withthe alcoholic solution, preferably the magnesium oxide matrix is addedinto the alcoholic solution, to form a slurry. In preferred embodiments,a weight ratio of the polyamine to the magnesium oxide matrix duringthis wet impregnating step is 1:1 to 1:3, preferably 1:1.2 to 1:2.8,preferably 1:1.4 to 1:2.6, preferably 1:1.6 to 1:2.4, preferably 1:1.8to 1:2.2, preferably 1:2. The slurry may be optionally agitated (e.g.,stirred, shaken, sonicated, etc.) at 10 to 50° C., preferably 15 to 40°C., preferably 20 to 30° C., for 0.5 to 24 hours, preferably 1 to 12hours, preferably 1.5 to 6 hours, preferably 2 to 3 hours. Afterwards,the slurry or paste (some alcohol may evaporate) may be dried forexample in an oven at 50 to 80° C., preferably 55 to 75° C., preferably60 to 70° C., for 1 to 24 hours, preferably 5 to 20 hours, preferably 10to 16 hours, preferably 12 to 14 hours to form the aminated magnesiumoxide adsorbent.

CO₂ Capture and Hydrogen Storage Methods

The present disclosure also provides a method of capturing CO₂ from agas mixture with the aminated magnesium oxide adsorbent disclosedherein. The methods herein can be used for the capture of CO₂ from largepoint sources, such as large fossil fuel or biomass electricity powerplants, biogas upgrading facilities, industries with major CO₂emissions, natural gas processing, synthetic fuel plants, and fossilfuel-based hydrogen production plants. Capture from the open atmosphereis also possible. Therefore, the aminated magnesium oxide adsorbent maybe useful in CO₂ removal/capture from various gas mixtures that containcarbon dioxide (CO₂) and at least one other gas. The other gas mayinclude, but is not limited to, nitrogen, hydrogen, oxygen, water(vapor), carbon monoxide, hydrocarbons having 1-4 carbon atoms (e.g.methane, ethane, ethylene, acetylene, propane, propylene, butane,iso-butane), nitrogen oxides (i.e. nitric oxide, nitrous oxide, nitrogendioxide), and noble gases (e.g. helium, neon, argon, krypton, xenon),including mixtures thereof. In preferred embodiments, the other gas isone or more of hydrogen, oxygen, nitrogen, methane, and carbon monoxide,more preferably one or more of nitrogen and methane.

The aminated magnesium oxide adsorbent of the present disclosure may bewell-suited for applications where fossil fuels or other energy sourcesare burned for electricity. For example, the gas mixture may be apre-combustion gas mixture, that is, a gas mixture produced from a fuelsource prior to combustion taking place. Pre-combustion processing istypically used in the production of fertilizer, chemical gaseous fuel(H₂, CH₄), cement processing, and power production facilities (e.g.,biomass power plant), and the like.

For example, in gasification processes a feedstock (such as coal) ispartially oxidized in steam and oxygen/air under high temperature andpressure, for instance in a gasifier, to form synthesis gas. Thissynthesis gas, or syngas, is a mixture of hydrogen, carbon dioxide (CO₂)and smaller amounts of other gaseous components, such as methane. Syngasis an important intermediate for production of hydrogen, ammonia,methanol, and synthetic hydrocarbon fuels, and can be produced from manysources, including natural gas, coal, biomass, or virtually anyhydrocarbon feedstock, by reaction with steam (steam reforming), carbondioxide (dry reforming), or oxygen (partial oxidation). For example,syngas can be subject to the water-gas shift reaction to convert CO andwater to H₂ and CO₂, producing a H₂ and CO₂-rich gas mixture. The CO₂can then be captured and separated, transported, and ultimatelysequestered or processed, and the H₂-rich fuel combusted. Syngas is alsoused as an intermediate in producing synthetic petroleum for use as afuel or lubricant via the Fischer-Tropsch process and previously theMobil methanol to gasoline process.

In another example, the pre-combustion gas mixture may be a biogas(mostly CH₄, CO₂, and in some cases N₂), and the method of the presentdisclosure may be applied to biogas upgrading. Here, the biogas issubject to a cleaning process using the aminated magnesium oxideadsorbent whereby the carbon dioxide and any water, nitrogen, hydrogensulfide, and particulates are removed, if present, to produce biomethanewith acceptable pipeline purity for distribution networks to be used asfuel (combusted).

In preferred embodiments, the method is applied to remove/capture CO₂from a pre-combustion gas mixture (e.g., a biogas), for example apre-combustion gas mixture having a CO₂ concentration of 15 to 50 vol.%, preferably 20 to 45 vol. %, preferably 25 to 40 vol. %, preferably 30to 35 vol. %, based on a total volume of the (pre-combustion) gasmixture.

Alternatively, the gas mixture may be a post-combustion gas mixture,i.e., a gas mixture produced after combustion of a fossil fuel, forexample the gas mixture may be an exhaust (flue) gas from a powerstation or other large point source. In some embodiments, the method isapplied to remove/capture CO₂ from a post-combustion gas mixture, forexample a post-combustion gas mixture having a CO₂ concentration of 5 to15 vol. %, preferably 6 to 14 vol. %, preferably 7 to 13 vol. %,preferably 8 to 12 vol. %, preferably 9 to 11 vol. %, preferably 10 vol.%, based on a total volume of the (post-combustion) gas mixture.Additionally, the post-combustion gas mixture may also include 70 to 75vol. %, preferably 71 to 74 vol. %, preferably 72 to 73 vol. % of N₂ and5 to 7 vol. %, preferably 5.5 to 6.5 vol. %, preferably 6 vol. % H₂O,each based a total volume of the (post-combustion) gas mixture. In someembodiments, the CO₂-capturing methods herein are performedpost-combustion, i.e., the gas mixture is a post-combustion gas mixture,for example, a flue gas.

The CO₂ capture/removal methods of the present disclosure may beperformed by contacting the gas mixture with the aminated magnesiumoxide adsorbent disclosed herein to adsorb at least a portion of the CO₂into/onto the aminated magnesium oxide adsorbent, thereby forming aloaded aminated magnesium oxide adsorbent and a gas stream depleted inCO₂ compared to the gas mixture.

Adsorption technologies may be employed herein for CO₂ capture, forexample, the CO₂ may be adsorbed by the aminated magnesium oxideadsorbent via a physisorption process, meaning the process is primarilyphysical and preferably no permanent chemical changes occur on theaminated magnesium oxide adsorbent or to the CO₂ molecules. If chemicalchanges do occur, such changes are transient and reversible so thatdesorption may be achieved to form intact CO₂ molecules. As such, theaminated magnesium oxide adsorbent may be freestanding or supported onor within a substrate, for example, the aminated magnesium oxideadsorbent may be housed within a chamber, for example, a column, plug,or filter, and/or on/within a substrate such as silica, alumina, and thelike. Preferably, the aminated magnesium oxide adsorbent may besupported within a fixed-bed column.

The chamber may be of any shape so long as the aminated magnesium oxideadsorbent can be securely housed and utilized inside the chamber toaccomplish the gas adsorption. The chamber may include an inletconfigured to accept a feed stream (gas mixture), a gas stream outletconfigured to expel a permeate (a gas stream depleted in CO₂), andoptionally a retentate outlet configured to expel a retentate (a CO₂rich stream). The chamber can be configured to be pressurized so as toforce the gas mixture though the inlet and through a bed of the aminatedmagnesium oxide adsorbent (and optionally a support) to enable infusionof CO₂ present in the gas mixture into the pore spaces of the aminatedmagnesium oxide adsorbent, thereby forming the loaded aminated magnesiumoxide adsorbent. The chamber may also be connected to a vacuum pump toprovide vacuum or a reduced pressure to the gas stream outlet for asimilar purpose.

Membrane gas separation technologies may also be employed herein for CO₂capture, for example, the aminated magnesium oxide adsorbent may beutilized in a mixed matrix membrane by homogeneously interpenetratingthe aminated magnesium oxide adsorbent of the present disclosure withina polymer matrix, along with other optional filler materials. In suchcases, the aminated magnesium oxide adsorbent may be present in anamount of 0.1 to 50 wt. %, preferably 0.5 to 40 wt. %, preferably 1 to30 wt. %, preferably 2 to 20 wt. %, preferably 3 to 15 wt. %, preferably4 to 10 wt. %, preferably about 5 wt. %, relative to a total weight ofthe membrane.

The membrane may be a thin film membrane (e.g., a thickness of 10 to2,000 μm), a flat sheet membrane, a spiral membrane, a tubular membrane,or a hollow fiber membrane. The membrane may be in the form of variousshapes, for example, flat (e.g., for a disc-shaped membrane), bent,curved (e.g., a cylinder shaped membrane), and rippled. The membrane mayhave a porous morphology. For example, the membrane may containunconnected pores each representing an isolated cavity having anunbroken pore wall, with the pores extending through the membranewithout intersecting one another (e.g., monolithic membrane).Alternatively, the membrane may contain pores which are part of aninterconnected network of pores where the pores in the membrane arerandomly oriented and intersect. The membrane may contain micropores (anaverage diameter of less than 2 nm), mesopores (an average diameter of2-50 nm), macropores (an average diameter larger than 50 nm), or amixture thereof. For example, the membrane may be macroporous, havingpores with an average diameter in a range of 0.5 to 10 μm, preferably 1to 8 μm, preferably 1.5 to 6 μm, preferably 2 to 5 μm, preferably 3 to 4μm.

The polymer matrix preferably has a high glass transition temperature(T_(g)), high melting point, and high crystallinity, i.e., the polymeris preferably a rigid, glassy polymer. In some embodiments, the polymer(of the polymer matrix) has a weight average molecular weight (M_(w)) of1×10⁴ to 2×10⁷ g/mol, preferably 5×10⁴ to 1.5×10⁷ g/mol, preferably1×10⁵ to 1×10⁷ g/mol.

Exemplary polymers that may be used to construct the polymer matrix inthe disclosed mixed matrix membranes include, but are not limited to:

-   -   polyolefins such as polyethylene, polypropylene, polybutene-1,        and poly(4-methyl pentene-1), including polyvinyls and        fluoropolymer variants thereof, for example polyvinylidene        fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl        chloride, polyvinyl fluoride, polyvinylidene chloride,        polyvinylidene fluoride, polyvinyl alcohol, polyvinyl ester        (e.g., polyvinyl acetate and polyvinyl propionate), polyvinyl        pyridine, polyvinyl pyrrolidone, polyvinyl ether, polyvinyl        ketone, polyvinyl aldehyde (e.g., polyvinyl formal and polyvinyl        butyral), polyvinyl amide, polyvinyl amine, polyvinyl urethane,        polyvinyl urea, polyvinyl phosphate, and polyvinyl sulfate;    -   polystyrene (e.g., isotactic polystyrene and syndiotactic        polystyrene), including styrene-containing copolymers such as        acrylonitrilestyrene copolymers, styrene-butadiene copolymers        and styrene-vinylbenzylhalide copolymers;    -   thermoplastic elastomers (TPE);    -   silicones such as polydimethylsiloxane (PDMS) and        polymethylphenylsilicone (PMPS);    -   polyacetylenes such as polytrimethylsilylpropyne;    -   polysulfones including polyethersulfones (PESs) as well as        sulfonated PESs, with specific mention being made to        poly(1,4-phenylene ether-ether-sulfone),        poly(1-hexadecene-sulfone), poly(1-tetradecene-sulfone),        poly(oxy-1,4phenylenesulfonyl-1,4-phenylene),        poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),        poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),        poly(oxy-1,4-phenylenesulfonyl-1,4-phenylene),        polyphenylsulfone, and ULTRASON S 6010 from BASF;    -   polysulfonamides such as        poly[1-[4-(3-carboxy-hydroxyphenylazo)benzenesulfonamido]-1,2-ethanediyl]);    -   polyacetals;    -   polyethers;    -   polyethylenimines;    -   polycarbonates;    -   cellulosic polymers such as cellulose acetate, cellulose        triacetate, cellulose acetate-butyrate, cellulose propionate,        ethyl cellulose, methyl cellulose, and nitrocellulose;    -   polyamides including aromatic polyamides and aliphatic        polyamides, such as Nylon 6 and polyphthalamide;    -   polyimides with specific mention being made to KAPTON (poly        (4,4′-oxydiphenylene-pyromellitimide) by DuPont, MATRIMID by        Huntsman Advanced Materials, P84 by HP Polymers GmbH,        poly(3,3′,4,4′-benzophenone tetracarboxylic        dianhydride-pyromellitic        dianhydride-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) (or        poly(BTDA-PMDA-TMMDA)), poly(3,3′,4,4′-benzophenone        tetracarboxylic dianhydride-pyromellitic        dianhydride-4,4′-oxydiphthalic        anhydride-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) (or        poly(BTDA-PMDA-ODPA-TMMDA)), poly(3,3′,4,4′-diphenylsulfone        tetracarboxylic dianhydride-3,3′,5,5′-tetramethyl-4,4′-methylene        dianiline) (or poly(DSDA-TMMDA)), poly(3,3′,4,4′-benzophenone        tetracarboxylic dianhydride-3,3′,5,5′-tetramethyl-4,4′-methylene        dianiline) (or poly(BTDA-TMMDA)), poly(3,3′,4,4′-diphenylsulfone        tetracarboxylic dianhydride-pyromellitic        dianhydride-3,3′,5,5′-tetramethyl-4,4′-methylene dianiline) (or        poly(DSDA-PMDA-TMMDA)), and poly[2,2′-bis-(3,4-dicarboxyphenyl)        hexafluoropropane        dianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane]        (or poly(6FDA-APAF)), poly[2,2′-bis-(3,4-dicarboxyphenyl)        hexafluoropropane        dianhydride-2,4,6-trimethyl-1,3-phenylenediamine] (or        poly(6FDA-DAM), poly[3,3′,4,4′-benzophenonetetracarboxylic        dianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane]        (or poly(BTDA-APAF)), poly(3,3′,4,4′-benzophenonetetracarboxylic        dianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl) (or        poly(BTDA-HAB)), poly[4,4′-oxydiphthalic        anhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane]        (or poly(ODPA-APAF)), poly[3,3′,4,4′-diphenylsulfone        tetracarboxylic        dianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane]        (or poly(DSDA-APAF)), poly(3,3′,4,4′-diphenylsulfone        tetracarboxylic        dianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl) (or        poly(DSDA-HAB)), poly[2,2′-bis-(3,4-dicarboxyphenyl)        hexafluoropropane        dianhydride-3,3′,4,4′-benzophenonetetracarboxylic        dianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane]        (or poly(6FDA-BTDA-APAF)), poly[4,4′-oxydiphthalic        anhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl]        (or poly(ODPA-APAF-HAB)),        poly[3,3′,4,4′-benzophenonetetracarboxylic        dianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane-3,3′-dihydroxy-4,4′-diamino-biphenyl]        (or poly(BTDA-APAF-HAB)), poly[2,2′-bis-(3,4-dicarboxyphenyl)        hexafluoropropane        dianhydride-3,3′-dihydroxy-4,4′-diamino-biphenyl] (or        poly(6FDA-HAB)), and poly(4,4′-bisphenol A        dianhydride-3,3′,4,4′-benzophenonetetracarboxylic        dianhydride-2,2-bis(3-amino-4-hydroxyphenyl)-hexafluoropropane)        (or poly(BPADA-BTDA-APAF));    -   polyetherimides such as ULTEM products manufactured by Sabic        Innovative Plastics;    -   polyamide imides;    -   polyketones;    -   polyether ketones such as polyether ether ketone, sulfonated        polyether ether ketone and the like;    -   polyarylene oxides such as polyphenylene oxide, polyxylene        oxide, sulfonated polyxylene oxide and brominated polyxylene        oxide;    -   polyurethanes;    -   polyureas;    -   polyazomethines;    -   polyesters including polyarylates such as polyethylene        terephthalate and polyphenylene terephthalate;    -   acrylates such as polyalkyl (meth)acrylate, polyacrylate,        polyacrylate-polyacrylamide copolymers;    -   polysulfides;    -   heterocyclic thermoplastics such as polybenzimidazoles,        polyoxadiazoles, polytriazoles, polybenzoxazole, and        polybenzimidazole;    -   polycarbodiimides;    -   polyphosphazines;    -   polyhydrazides;    -   and copolymers thereof, including block copolymers, grafts, and        blends thereof

The mixed matrix membrane may be made by methods known to those ofordinary skill in the art, for example, by casting or melt blending, andthe polymer matrix may be made porous by known techniques including, butnot limited to, irradiation, stretching of a melt-processedsemi-crystalline polymer substrate, vapor-induced phase separation, andtemperature-induced phase separation, just to name a few.

When the aminated magnesium oxide adsorbent of the present disclosure isutilized in mixed matrix membrane separation technologies, the membranemay be housed in chamber such that the membrane divides the chamber intoa feed side and a permeate side. The gas mixture may then be fed intothe feed side of the chamber so that at least a portion of the CO₂present in the gas mixture permeates the membrane and is adsorbed by theaminated magnesium oxide adsorbent, thereby forming the loaded aminatedmagnesium oxide adsorbent. This may be accomplished for example bysupplying the gas mixture at above atmospheric pressure or otherwiseforcing at least a portion of the gas mixture through the membrane bypressurizing the feed side, or applying a vacuum to the permeate side ofthe chamber. A gas stream depleted in CO₂ compared to the gas mixturemay then be collected from the permeate side, and the chamber may beoptionally configured to include a retentate outlet to expel a retentate(a CO₂ rich stream) after desorbing CO₂ molecules from the loadedaminated magnesium oxide adsorbent.

Regardless of whether an adsorptive technique (e.g., fixed-bed of theaminated magnesium oxide adsorbent) or a membrane gas separationtechnology is utilized, the gas mixture is contacted with the aminatedmagnesium oxide adsorbent disclosed herein. The gas mixture may becontacted with the aminated magnesium oxide adsorbent in a singlechamber, or the gas mixture may be passed through a series of chambershousing the aminated magnesium oxide adsorbent to gradually/sequentiallyremove/capture CO₂ from the gas mixture. Similarly, the aminatedmagnesium oxide adsorbent may be used in addition to other knownadsorption materials to provide a gas stream depleted in CO₂ and aloaded aminated magnesium oxide adsorbent.

In some embodiments, prior to contacting the gas mixture with theaminated magnesium oxide adsorbent, the aminated magnesium oxideadsorbent may be activated through a degassing procedure performed in asub-atmospheric pressure of 0.05 to 0.8 atm, preferably 0.1 to 0.5 atm,preferably 0.2 to 0.4 atm to remove gas or solvent molecules that mayreside in the pore spaces of the aminated magnesium oxide adsorbent. Theaminated magnesium oxide adsorbent may be degassed at a temperature of 0to 200° C., preferably 10 to 150° C., preferably 25 to 100° C., or about80° C. for 1 to 48 hours, preferably 2 to 36 hours, preferably 8 to 24hours, preferably 12 to 18 hours.

A force may be provided to deliver the gas mixture into contact with theaminated magnesium oxide adsorbent. The gas mixture may be introduced ata flow rate of 0.001 to 1,000 L/min, preferably 0.005 to 500 L/min,preferably 0.01 to 100 L/min, preferably 0.05 to 10 L/min, preferably0.1 to 5 L/min, preferably 0.5 to 2 L/min. In some embodiments, the gasmixture is pressurized (e.g., be applying pressure to a feed side of achamber) at a pressure of greater than 760 and up to 4,000 Torr,preferably 800 to 3,500 Torr, preferably 850 to 3,000 Torr, preferably900 to 2,500 Torr, preferably 1,000 to 2,000 Torr to force at least aportion of the gas mixture to contact the aminated magnesium oxideadsorbent. In some embodiments, the gas mixture is contacted with theaminated magnesium oxide adsorbent under vacuum, for example by applyinga reduced pressure of less than 760 Torr, preferably 10 to 750 Torr,preferably 20 to 700 Torr, preferably 30 to 600 Ton to the permeate sideof a chamber such that at least a portion of the gas mixture is broughtinto contact with the aminated magnesium oxide adsorbent. In preferredembodiments, the gas mixture is contacted with the aminated magnesiumoxide adsorbent under vacuum at a reduced pressure of 60 to 160 Torr,preferably 70 to 150 Torr, preferably 80 to 140 Torr, preferably 90 to130 Torr, preferably 100 to 120 Torr. Alternatively, the gas mixture maystay stagnant over the aminated magnesium oxide adsorbent (i.e. as anatmosphere to the aminated magnesium oxide adsorbent) for a suitableamount of time to enable adsorption of CO₂.

The gas mixture may be contacted with the aminated magnesium oxideadsorbent at any temperature that enables desired levels of CO₂ capture,for example, the gas mixture may have a temperature of −5 to 80° C.,preferably 0 to 75° C., preferably 5 to 60° C., preferably 10 to 50° C.,preferably 20 to 35° C., preferably 25 to 30° C. In preferredembodiments, desirable levels of CO₂ capture can be achieved with theaminated magnesium oxide adsorbent at ambient conditions, for example 20to 25° C. and about 1 atm.

A gas stream depleted in CO₂ may be obtained after at least a portion ofCO₂ is adsorbed onto the aminated magnesium oxide adsorbent. Acomposition of the gas stream depleted in CO₂ may vary depending on thecomposition of the gas mixture. In some embodiments, the gas streamdepleted in CO₂ contains at least 25% less CO₂, preferably at least 30%less CO₂, preferably at least 40% less CO₂, preferably at least 50% lessCO₂, preferably at least 60% less CO₂, preferably at least 70% less CO₂,preferably at least 80% less CO₂, preferably at least 90% less CO₂,preferably at least 95% less CO₂, by volume compared to a volume of CO₂present in the gas mixture. For example, when the methods herein areemployed in pre-combustion processes, the gas stream depleted in CO₂ maycontain less than 35 vol % CO₂, preferably less than 25 vol % CO₂,preferably less than 20 vol % CO₂, preferably less than 15 vol % CO₂,preferably less than 10 vol % CO₂, preferably less than 5 vol % CO₂,preferably less than 1 vol % CO₂, preferably less than 0.5 vol % CO₂,preferably less than 0.1 vol % CO₂, based on a total volume of gasstream depleted in CO₂ When the methods herein are employed inpost-combustion processes, the gas stream depleted in CO₂ may containless than 10 vol % CO₂, preferably less than 8 vol % CO₂, preferablyless than 6 vol % CO₂, preferably less than 4 vol % CO₂, preferably lessthan 2 vol % CO₂, preferably less than 1 vol % CO₂, preferably less than0.5 vol % CO₂, preferably less than 0.1 vol % CO₂, preferably less than0.05 vol % CO₂, preferably less than 0.01 vol % CO₂, based on a totalvolume of gas stream depleted in CO₂.

In some embodiments, the method of the present disclosure furtherinvolves desorbing the CO₂ from the loaded aminated magnesium oxideadsorbent, and reusing the aminated magnesium oxide adsorbent. Thecarbon dioxide may be stripped off the aminated magnesium oxideadsorbent using temperature swing adsorption (TSA) or pressure swingadsorption (PSA) techniques so the aminated magnesium oxide adsorbentcan be reused. For instance, desorbing the CO₂ may involve heating theloaded aminated magnesium oxide adsorbent at a temperature of 50 to 200°C., preferably 60 to 180° C., preferably 70 to 160° C., preferably 80 to140° C., preferably 90 to 120° C., preferably 100 to 110° C., subjectingthe loaded aminated magnesium oxide adsorbent to a reduced pressure ofless than 750 Torr, preferably less than 700 Torr, preferably less than600 Torr, preferably less than 500 Torr, preferably less than 400 Torr,preferably less than 300 Torr, preferably less than 200 Torr, preferablyless than 100 Torr, preferably less than 75 Torr, preferably less than50 Torr, preferably less than 25 Torr, or a combination of heat andreduced pressure. The loaded aminated magnesium oxide adsorbent may alsobe regenerated (by passing a nitrogen gas stream over the loadedaminated magnesium oxide adsorbent. For example, a nitrogen gas streammay be flowed over the loaded aminated magnesium oxide adsorbent at aflow rate of 0.001 to 1,000 L/min, preferably 0.005 to 500 L/min,preferably 0.01 to 100 L/min, preferably 0.05 to 10 L/min, preferably0.1 to 5 L/min, preferably 0.5 to 2 L/min, at a temperature of 50 to200° C., preferably 60 to 180° C., preferably 70 to 160° C., preferably80 to 140° C., preferably 90 to 120° C., for 1 to 60 minutes, preferably5 to 50 minutes, preferably 10 to 40 minutes, preferably 20 to 35minutes, preferably 30 minutes in order to desorb CO₂.

The loaded aminated magnesium oxide adsorbent may be regenerated (i.e.desorbed) and reused without a significant loss in CO₂ uptake capacity.For instance, the aminated magnesium oxide adsorbent may be used tocapture CO₂, desorbed, and reused for up to 25 cycles, preferably up to20 cycles, preferably up to 15 cycles, preferably up to 10 cycles,preferably up to 5 cycles, and the CO₂ uptake capacity may be reduced byno more than 10%, preferably no more than 8%, preferably no more than6%, preferably no more than 4%, preferably no more than 3%, preferablyno more than 2%, preferably no more than 1%, relative to an initial CO₂uptake capacity of the aminated magnesium oxide adsorbent.

Desorbing the CO₂ from the loaded aminated magnesium oxide adsorbentgenerates a gas stream enriched in CO₂. Such a gas stream may beoptionally subjected to further processing steps such as an additionalpurification step (e.g. column chromatography, further membraneseparation steps, etc.), and any captured and collected CO₂ mayoptionally be subject to numerous processing steps, for example, usedfor the production of urea, methanol, metal carbonates and bicarbonates,aromatic and aliphatic polycarbonates, and sodium salicylate, as well asused in biotransformations to form fuels such as isobutyraldehyde andisobutanol, as is known to those of ordinary skill in the art.

In addition to pre-combustion and/or post-combustion CO₂ captureapplications, it is contemplated that the aminated magnesium oxideadsorbents disclosed herein may be used in air purifiers, chemicalfilters, oil and gas refineries, fermenters, bioreactors, or in anyother process where the capture/removal of CO₂ is desired.

In preferred embodiments, the methods herein rely on the adsorbentproperties of the aminated magnesium oxide adsorbent, and no othersieving/support materials, such as those listed previously (e.g.,hierarchical mesoporous silicates and aluminosilicates, carbonaceousmaterials, and molecular organic frameworks), are utilized for CO₂capture.

The examples below are intended to further illustrate protocols forpreparing the aminated magnesium oxide adsorbents and for using theaminated magnesium oxide adsorbents in CO₂ capture applications, and arenot intended to limit the scope of the claims.

Where a numerical limit or range is stated herein, the endpoints areincluded. Also, all values and subranges within a numerical limit orrange are specifically included as if explicitly written out.

The terms “comprise(s)”, “include(s)”, “having”, “has”, “contain(s)”,and variants thereof, as used herein, are intended to be open-endedtransitional phrases, terms, or words that do not preclude thepossibility of additional acts or structures. The present disclosurealso contemplates other embodiments “comprising”, “consisting of” and“consisting essentially of”, the embodiments or elements presentedherein, whether explicitly set forth or not.

As used herein the words “a” and “an” and the like carry the meaning of“one or more.”

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeto be understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically describedherein.

All patents and other references mentioned above are incorporated infull herein by this reference, the same as if set forth at length.

EXAMPLES

Materials

Magnesium nitrate ≥99%, ammonium hydroxide (30%), oxalic acid, sodiumhydroxide, ethanol, diethylenetriamine (DETA), and polyethylene imine(PEI) with a molecular weight of 10,000 g/mol were used to prepareMgO-based adsorbents with different characteristics. All chemicals usedwere of an analytical grade or above; the chemicals were obtained fromSigma-Aldrich and used as received.

Preparation of Porous MgO Using Ammonium Hydroxide Route (MgO-A)

In this approach, MgO was synthesized first by dissolving 7 g ofmagnesium nitrate hexahydrate (Mg(NO₃)₂.6H₂O) in 100 mL distilled water.After that, the mixture was stirred at room temperature until ahomogenous magnesium nitrate solution was formed (˜5 min). Then, anammonium hydroxide (NH₄OH) solution (25 wt. % in water) was added to themagnesium nitrate solution to make the molar ratio of NH₄OH:Mg as 5:1.The addition of NH₄OH was followed by stirring for 5 min at roomtemperature. Then, the flask was tightly closed (to prevent ammoniumhydroxide evaporation) and slowly heated in a water bath until thetemperature reached 60° C. The mixture was maintained at thistemperature under continuous stirring for 6 h. Then, the solution wasremoved from the water bath and kept at room temperature while stirringfor another 24 h. The flask was kept open during this 24-h stirringprocess. The continuous stirring resulted in the formation of a thickpaste (consists of Mg(OH)₂ and NH₄NO₃ in water) according to thefollowing reaction (A. Hanif, S. Dasgupta, A. Nanoti, Facile Synthesisof High-Surface-Area Mesoporous MgO with Excellent High-Temperature CO₂Adsorption Potential, Industrial & Engineering Chemistry Research, 55(2016) 8070-8078, incorporated herein by reference in their entirety):Mg(NO₃)₂+2NH₄OH→Mg(OH)₂+2NH₄NO₃   (1)

The paste was then washed with distilled water several times to removeNH₄NO₃. The dissolved NH₄NO₃ in water was removed through repeated 5-mincentrifugation at 4500 rpm. This extensive wash would also remove anyunreacted Mg(NO₃)₂.6H₂O. The produced Mg(OH)₂ was dried overnight in anoven at 60° C. The obtained solid was ground and then calcined for 10 hat 400° C. under air to yield MgO according to the following equation:Mg(OH)₂→MgO+H₂OThe produced MgO using this preparation method (ammonium hydroxide) wasdesignated as “MgO-A”. Calcination at 300 and 500° C. was also conductedseparately to probe the effect of the calcination temperature on themorphology, crystallinity, and porosity of MgO, in addition to its CO₂adsorption capacity, which will be discussed later.Preparation of Porous MgO Using An Oxalic Acid Route (MgO—O)

In this synthesis, 5.6 g of Mg(NO₃)₂.6H₂O and 2 g of oxalic acid(C₂H₂O₄) were dissolved separately in 25 mL ethanol. Then, a mixture ofequimolar concentrations of Mg(NO₃)₂.6H₂O and C₂H₂O₄ was prepared bymixing the above two solutions. The resultant mixture was stirredvigorously at room temperature for 12 h to ensure the completion of thereaction. The products of the reaction between Mg(NO₃)₂.6H₂O and C₂H₂O₄is a white gel consisting of magnesium oxalate (hydrate) (MgC₂O₄.2H₂O),nitric acid (HNO₃) and water according to the following reaction:Mg(NO₃)₂.6H₂O+(COOH)₂.2H₂O→MgC₂O₄.2H₂O+2HNO₃+6H₂O   (2)See A. Kumar, J. Kumar, On the synthesis and optical absorption studiesof nano-size magnesium oxide powder, Journal of Physics and Chemistry ofSolids, 69 (2008) 2764-2772, incorporated herein by reference in itsentirety.

At this stage, the obtained white gel was first dried at 100° C. for 24h to evaporate the formed nitric acid and water. Finally, the obtainedwhite solid was ground and then it was calcined at 400° C. for 10 hunder air, and the obtained magnesium oxide was labeled as “MgO—O”.

Preparation of Porous MgO Using Sodium Hydroxide Route (MgO—N)

In this MgO preparation method, about 5.13 g of Mg(NO₃)₂.6H₂O wasdissolved in 100 mL distilled water to yield 0.2 M solution. To thissolution, 0.5 M sodium hydroxide (NaOH) was added drop-wise, whilestirring, until the pH of the solution reached 12.5. Then, the mixturewas stirred at room temperature for 30 min. The reaction betweenMg(NO₃)₂.6H₂O and NaOH produces a white precipitate of Mg(OH)₂ andsodium nitrate (NaNO₃) according to the following reaction:Mg(NO₃)₂+2NaOH→Mg(OH)₂+2NaNO₃   (3)See R. Wahab, S. G. Ansari, M. A. Dar, Y. S. Kim, H. S. Shin, Synthesisof Magnesium Oxide Nanoparticles by Sol-Gel Process, Materials ScienceForum, 558-559 (2007) 983-986, incorporated herein by reference in itsentirety.

The sodium nitrate salt was removed through an extensive wash withethanol and distilled water; Mg(OH)₂ was removed from the sodium nitratesolution via centrifugation at 4500 rpm for 5 min after every wash withethanol or distilled water. After the last wash, Mg(OH)₂ was dried at60° C. overnight. The obtained solid was ground before subjecting it to10-h calcination at 400° C. under air. The obtained magnesium oxide wasdesignated as “MgO—N”.

Functionalization of MgO with DETA and PEI

The functionalization of MgO with DETA and PEI was achieved using thewet impregnation method, described elsewhere, with some modifications.See X. Xu, C. Song, J. M. Andresen, B. G. Miller, A. W. Scaroni,Preparation and characterization of novel CO₂ “molecular basket”adsorbents based on polymer-modified mesoporous molecular sieve MCM-41,Microporous and Mesoporous Materials, 62 (2003) 29-45, incorporatedherein by reference in its entirety. The first step in such afunctionalization protocol was the preparation of an ethanolic solutionof each amine by mixing 0.25 g of either DETA or PEI with 25 mL ethanol,followed by stirring at room temperature for 15 min. Then, 500 mg of thesynthesized MgO-A was added to each amine solution, and the resultantslurries were stirred continuously for 2 h. After that, the formedpastes (some ethanol evaporates during mixing) were dried at 60° C.overnight. The obtained samples were labeled as DETA-MgO-A andPEI-MgO-A.

Characterization of the Synthesized Adsorbents

The synthesized MgO-based adsorbents were characterized using differenttechniques. The BET (Brunauer-Emmett-Teller) surface area of MgO-basedadsorbents (including amine-functionalized MgO) was obtained through themeasurements of nitrogen adsorption/desorption isotherms at −196° C.using an ASAP 2020 system (Micromeritics Instruments, Inc.). Themorphologies of the synthesized MgO-based adsorbents and their elementalcompositions were determined using a scanning electron microscopy (SEM)coupled with an energy-dispersive X-ray spectroscopy (EDX) technique.The crystallinity of the synthesized MgO-based adsorbents was studiedusing XRD pattern by XRD-6000 X-ray diffractometer with Curadiation 40kV and 30 mA. Functional groups in the synthesized adsorbents wereidentified using Fourier Transform Infrared (FTIR) spectra (NICOLET 6700spectrometer, Thermo Scientific) and recorded at room temperature in awavenumber ranging from 400 to 4000 cm⁻¹ using KBr pellets.

Evaluation of CO₂ Adsorption on MgO-Based Adsorbents

The measurements of CO₂ uptake capacity by the synthesized MgO-basedadsorbents were carried out using the thermogravimetric analysistechnique (TGA Q600 SDT). In each run, about 10 mg of one thesynthesized adsorbents was placed on an alumina oxide pan and, then, theadsorbent was degassed at 120° C. using a stream of pure N₂ gas flowingat 100 mL/min. Once the weight of the sample reached a steady value(i.e., all water is removed), the temperature was dropped to 30° C. andleft to equilibrate. After the stabilization of the adsorptiontemperature at 30° C., the valve of the carbon dioxide (99.9% pure)cylinder was opened, and the flow rate of CO₂ was kept constant at 100mL/min. The adsorption pressure was 1 atm. The adsorption of CO₂ wasmonitored in time until the adsorption approached equilibrium (within 90min contact between CO₂ and each adsorbent). While the above procedurewas run with a pure CO₂ gas, any other gas mixture containing CO₂ can beused instead.

To assess the stability and regenerative properties of the DETA-MgO-Aadsorbent, this adsorbent was regenerated by conducting 4 cycles ofconsecutive adsorption and regeneration. Regeneration was performedusing pure N₂ gas flowing at 100 mL/min. The regeneration temperatureand time were 120° C. and 30 min. The regeneration experiments of thespent adsorbent were carried out at atmospheric pressure (˜1 atm).DETA-MgO-A was fully regenerated within less than 20 minutes of contactwith the N₂ gas stream at the above regeneration conditions.

Characterization of the Synthesized Adsorbents

The synthesized MgO-based adsorbents were characterized using FTIR inorder to confirm the formation of MgO and to get insights into theeffects of different preparation methods, the calcination temperature,and the amine functionalization chemical properties of MgO. FIG. 2presents the FTIR spectra of the parent (unmodified) MgO prepared usingammonium hydroxide (MgO-A). The spectral patterns of MgO-A, MgO—O, andMgO—N are comparable. The strong peaks appearing between 400 and 700cm⁻¹ correspond to the metal-oxygen bending vibration, confirming theformation of magnesium oxide. See I. F. Mironyuk, V. M. Gun'ko, M. O.Povazhnyak, V. I. Zarko, V. M. Chelyadin, R. Leboda, J.Skubiszewska-Zięba, W. Janusz, Magnesia formed on calcination of Mg(OH)₂prepared from natural bischofite, Applied Surface Science, 252 (2006)4071-4082; and G. Song, S. Ma, G. Tang, X. Wang, Ultrasonic-assistedsynthesis of hydrophobic magnesium hydroxide nanoparticles, Colloids andSurfaces A: Physicochemical and Engineering Aspects, 364 (2010) 99-104,each incorporated herein by reference in their entirety. The sharp andintense peaks at 3700 and 1650 cm⁻¹ and the broad peaks centered around3400 and 1450 cm⁻¹ are associated with the presence of water moistureand/or the presence of some residual surface hydroxyl groups (—OH). SeeG. Song, S. Ma, G. Tang, X. Wang, Ultrasonic-assisted synthesis ofhydrophobic magnesium hydroxide nanoparticles, Colloids and Surfaces A:Physicochemical and Engineering Aspects, 364 (2010) 99-104; and M.Rezaei, M. Khajenoori, B. Nematollahi, Synthesis of high surface areananocrystalline MgO by pluronic P123 triblock copolymer surfactant,Powder Technology, 205 (2011) 112-116, each incorporated herein byreference in their entirety. The minor peak appearing at 1100 cm⁻¹ forthe MgO—O sample might be attributed to C—O stretching vibration. Thereason for detecting this peak is likely due to the presence of someimpurities remained from the starting materials (i.e., oxalic acid). SeeM. Rezaei, M. Khajenoori, B. Nematollahi, Synthesis of high surface areananocrystalline MgO by pluronic P123 triblock copolymer surfactant,Powder Technology, 205 (2011) 112-116; and F. Meshkani, M. Rezaei,Facile synthesis of nanocrystalline magnesium oxide with high surfacearea, Powder Technology, 196 (2009) 85-88, each incorporated herein byreference in their entirety.

Despite the comparable spectral patterns of MgO-A, MgO—O, and MgO—N,their CO₂ adsorption capacities were reasonably different, to bepresented and discussed later, with MgO-A adsorbing more CO₂ than theother two. Thus, the effect of the calcination temperature andamine-functionalization are only reported for MgO-A. As was the casewith the different MgO synthesis routes, different calcinationtemperatures did not significantly alter the FTIR spectral pattern ofthe metal oxide, with the exception of the minor change in the peakintensity at 3700 cm⁻¹, related to water moisture and/or the presence ofsome residual surface hydroxyl groups (—OH). This peak becomes moreintense at the higher calcination temperature of 500° C. The decrease inthe peak intensity at 500° C. might be attributed to the removal of morehydroxyl groups at high calcination temperatures.

FIG. 2 also shows the FTIR spectra of MgO-A (prepared from ammoniumhydroxide route and calcined at 400° C., which provides the highest CO₂adsorption capacity as will be presented and discussed later) afterfunctionalization with DETA (DETA-MgO-A). The functionalization of MgO-Awith DETA or PEI resulted in the appearance of new peaks at 2960 and2840 cm⁻¹, which are attributed to the C—H bonds of the amine molecules.The bands which appeared between 1560 and 1300 cm⁻¹ are associated withthe NH and NH₂ deformation. See J. Wang, H. Chen, H. Zhou, X. Liu, W.Qiao, D. Long, L. Ling, Carbon dioxide capture usingpolyethylenimine-loaded mesoporous carbons, Journal of EnvironmentalSciences, 25 (2013) 124-132; Z. Zhang, B. Wang, Q. Sun, Fly Ash-derivedSolid Amine Sorbents for CO2 Capture from Flue Gas, Energy Procedia, 63(2014) 2367-2373; H. Y. Huang, R. T. Yang, D. Chinn, C. L. Munson,Amine-Grafted MCM-48 and Silica Xerogel as Superior Sorbents for AcidicGas Removal from Natural Gas, Industrial & Engineering ChemistryResearch, 42 (2003) 2427-2433; S.-M. Hong, S. H. Kim, a. K. B. Lee,Adsorption of Carbon Dioxide on 3-Aminopropyl-Triethoxysilane ModifiedGraphite Oxide, energy & fuels, 27 (2013) 3358-3363; and J. Pokhrel, N.Bhoria, S. Anastasiou, T. Tsoufis, D. Gournis, G. Romanos, G. N.Karanikolos, CO₂ adsorption behavior of amine-functionalized ZIF-8,graphene oxide, and ZIF-8/graphene oxide composites under dry and wetconditions, Microporous and Mesoporous Materials, 267 (2018) 53-67, eachincorporated herein by reference in their entirety.

In order to assess the crystallinity of the synthesized MgO samples, XRDanalysis was performed, and the results for MgO-A are shown in FIG. 1.The unmodified MgO samples (MgO-A, MgO—O, and MgO—N) have comparablepatterns. Each sample exhibits 5 diffraction peaks at 36.8°, 42.2°,61.2°, 74.1°, and 77.9° that can be, respectively, assigned to theplanes (111), (200), (220), (311) and (222) of cubic MgO, confirming itscrystalline structure. These results are in a good agreement with thereported observations in the related literature. See Y.-D. Ding, G.Song, X. Zhu, R. Chen, Q. Liao, Synthesizing MgO with a high specificsurface for carbon dioxide adsorption, RSC Advances, 5 (2015)30929-30935; N. Rani, S. Chahal, A. S. Chauhan, P. Kumar, R. Shukla, S.K. Singh, X-ray Analysis of MgO Nanoparticles by Modified Scherer'sWilliamson-Hall and Size-Strain Method, Materials Today: Proceedings, 12(2019) 543-548; and N. Yang, P. Ning, K. Li, J. Wang, MgO-basedadsorbent achieved from magnesite for CO₂ capture in simulate wet fluegas, Journal of the Taiwan Institute of Chemical Engineers, 86 (2018)73-80, each incorporated herein by reference in their entirety. Thepeaks intensity of the prepared MgO series at 42.2° decreased in theorder of MgO—O>MgO-A>MgO—N. These results demonstrate that the magnesiumoxide synthesized using the oxalic acid route has higher crystallinitythan those prepared using ammonium hydroxide and sodium hydroxide. SeeA. Hanif, S. Dasgupta, A. Nanoti, Facile Synthesis of High-Surface-AreaMesoporous MgO with Excellent High-Temperature CO₂ Adsorption Potential,Industrial & Engineering Chemistry Research, 55 (2016) 8070-8078,incorporated herein by reference in their entirety. Upon thefunctionalization of MgO-A with DETA or PEI, new peaks appeared at˜50.2°, ˜58.7° and ˜72.2° as shown in FIG. 1 for DETA-MgO-A. Moreover,the intensity of the originally small peak of MgO-A at around 36.8° hassignificantly increased upon amine-functionalization. Furthermore, themain MgO-A peak at 42.2° has dropped in intensity quite significantly.This is also the case for the original MgO-A peak at 61.2°. The otherminor peaks of MgO-A at 74.1° and 77.9° have also vanished upon itsfunctionalization with DETA and PEI. The appearance of new peaks atother diffraction angles as well as the decreased intensity of the MgO-Apeaks at 42.2° and 61.2° confirm the attachments of the amine moleculesbetween the MgO layers. See X. Xu, C. Song, J. M. Andrésen, B. G.Miller, A. W. Scaroni, Preparation and characterization of novel CO₂“molecular basket” adsorbents based on polymer-modified mesoporousmolecular sieve MCM-41, Microporous and Mesoporous Materials, 62 (2003)29-45; and S.-M. Hong, S. H. Kim, a. K. B. Lee, Adsorption of CarbonDioxide on 3-Aminopropyl-Triethoxysilane Modified Graphite Oxide, energy& fuels, 27 (2013) 3358-3363, each incorporated herein by reference inits entirety.

In addition to studying the crystallinity of the MgO-based adsorbents,their textural properties (i.e., BET surface area, pore size, and porevolume) have been also investigated using the N₂-physisorption at 77 K.Table 1 summarizes the textural properties of the synthesized MgO-basedadsorbents. The International Union of Pure and Applied Chemistry(IUPAC) has classified nanoporous adsorbents, based on pore size, intothree main categories: macroporous (pore diameter >50 nm), mesoporous (2nm<pore diameter<50 nm) and microporous (pore diameter <2 nm). See Y.Liu, B. Sajjadi, W.-Y. Chen, R. Chatterjee, Ultrasound-assisted aminefunctionalized graphene oxide for enhanced CO₂ adsorption, Fuel, 247(2019) 10-18, incorporated herein by reference in its entirety.Accordingly, the DETA-MgO-A-based adsorbent synthesized here (seeTable 1) falls into the mesoporous category. However, the pore sizes ofthe unmodified magnesium oxide adsorbents is influenced by the synthesisroute used. MgO-A possesses the highest pore volume as a result ofhaving the highest surface area.

TABLE 1 BET surface area, avg. pore size, and avg. pore volume ofMgO-based adsorbents Surface area Avg. pore volume Pore size Samples(BET) (m²/g) (cm³/g) (nm) MgO—A 350.12 0.414 4.73 MgO—N 149.27 0.59816.03 MgO—O 124.08 0.546 17.59 DETA-MgO—A 91.42 0.203 8.87 PEI-MgO—A72.51 0.178 9.81

Of the unmodified MgO adsorbents tested, MgO-A has the highest surfacearea. Notably, all three unmodified MgO displayed higher surface areasthan those of MgO produced using templates. See M. Rezaei, M.Khajenoori, B. Nematollahi, Synthesis of high surface areananocrystalline MgO by pluronic P123 triblock copolymer surfactant,Powder Technology, 205 (2011) 112-116; and Y.-D. Ding, G. Song, X. Zhu,R. Chen, Q. Liao, Synthesizing MgO with a high specific surface forcarbon dioxide adsorption, RSC Advances, 5 (2015) 30929-30935, eachincorporated herein by reference in its entirety. Furthermore, the threeunmodified MgO adsorbents possess much higher surface areas thancommercial MgO. For instance, MgO-A possesses a surface area of 350.12m²/g, which is over 10 times higher than the reported value for thecommercial MgO, which is 32 m²/g. See Y.-D. Ding, G. Song, X. Zhu, R.Chen, Q. Liao, Synthesizing MgO with a high specific surface for carbondioxide adsorption, RSC Advances, 5 (2015) 30929-30935, incorporatedherein by reference in its entirety. Benchmarking the surface areas ofthe three unmodified MgO with their crystallinity obtained from the XRDanalysis reveals the inverse relationship between the surface area of anadsorbent and its crystallinity. See W. Gao, T. Zhou, B. Louis, Q. Wang,Hydrothermal Fabrication of High Specific Surface Area Mesoporous MgOwith Excellent CO2 Adsorption Potential at Intermediate Temperatures,Catalysts, 7 (2017) 116, incorporated herein by reference in itsentirety.

In order to investigate the effect of the functionalization of MgO withamines on its textural properties, the pore size, BET surface area, andpore volume of the amine-modified MgO-A have been measured. As shown inTable 1, all textural properties decreased upon functionalization withDETA and PEI, with the exception of the pore size. The reductions insurface area and pore volume upon the incorporation of the aminemolecules to magnesium oxide are likely due to the polyamines fillingsome pores. See X. Xu, C. Song, J. M. Andresen, B. G. Miller, A. W.Scaroni, Novel Polyethylenimine-Modified Mesoporous Molecular Sieve ofMCM-41 Type as High-Capacity Adsorbent for CO₂ Capture, Energy & Fuels,16 (2002) 1463-1469; and S.-H. Liu, W.-C. Hsiao, W.-H. Sie,Tetraethylenepentamine-modified mesoporous adsorbents for CO₂ capture:effects of preparation methods, Adsorption, 18 (2012) 431-437, eachincorporated herein by reference in their entirety. Accordingly, thesurface area of the parent MgO-A has dropped by a factor of 3.8 upon theintroduction of DETA, and the huge reduction in the surface area ofPEI-MgO-A can be attributed to the bulky size of the polymeric aminePEI. It is interesting to note that despite having a surface area thatis about 74% less than that of the unmodified MgO-A, the pore size ofDETA-MgO-A is about 87% higher.

Insights into the morphologies of the synthesized MgO-based adsorbentshave been obtained using SEM imaging. It is clear from the SEM imagesthat the particles of MgO-A are small in size with irregularplatelet-like morphology. Contrarily, MgO—N particles are bigger in sizethan MgO-A, with an irregular sphere-like morphology. The irregularshape is also evident for the MgO—O particles, which resemble grainswith a cubic-like morphology, and they are bigger in size than MgO—N andMgO-A. Such observations clearly demonstrate the impact of the chemistryof the MgO synthesis solution on its morphological properties even whenthe same magnesium salt (i.e., MgO precursor) is used. A similarobservation has been reported in the literature on the effect ofcatalyst preparation protocol on its morphological properties. See E.Alvarado, L. M. Tones-Martinez, A. F. Fuentes, P. Quintana, Preparationand characterization of MgO powders obtained from different magnesiumsalts and the mineral dolomite, Polyhedron, 19 (2000) 2345-2351,incorporated herein by reference in its entirety. The results of SEManalysis are in line with the confirmed phase crystal using XRD thatindicated MgO—O possesses the high crystallinity and has the largestparticles size of the unmodified MgO samples tested.

As stated above, the chemistry of MgO synthesis solution has asignificant influence on the product morphology. Thus, the effect ofamine functionalization on the morphological properties was alsoanalyzed. SEM images made clear that the morphology of the parent MgO-Awas dramatically altered upon amine-functionalization with DETA and PEI,probably due to the coverage of the metal oxide surface with the aminemolecules. Another observation that can be extracted from these imagesis the huge reduction in the porosity of the original MgO-A due to thefilling of its pores with the amine molecules. Furthermore, thereduction in porosity confirmed the reduction trend obtained from theBET analysis, reported above. These observations are also in line withthe reduction in surface area and porosity of graphene oxide upon itsfunctionalization with 3-aminopropyltriethoxysilane andtetraethylenepentamine. See S.-M. Hong, S. H. Kim, a. K. B. Lee,Adsorption of Carbon Dioxide on 3-Aminopropyl-Triethoxysilane ModifiedGraphite Oxide, energy & fuels, 27 (2013) 3358-3363; and Y. Zhang, Y.Chi, C. Zhao, Y. Liu, Y. Zhao, L. Jiang, Y. Song, CO₂ AdsorptionBehavior of Graphite Oxide Modified with Tetraethylenepentamine, Journalof Chemical & Engineering Data, 63 (2018) 202-207, each incorporatedherein by reference in their entirety.

The above characterization results have revealed useful information onthe synthesized MgO-based adsorbents. Particularly, the FTIR resultssuggested the formation of magnesium oxide and its subsequentmodification with amines. To support these results, elementalcompositions of the synthesized MgO-based adsorbents have been analyzedusing EDX, and the results are summarized in Table 2. According to theresults presented in Table 2, all samples contain Mg and O, confirmingthe formation of magnesium oxide. However, unlike the pure MgO-A andMgO—N, MgO—O contains 6.64 wt % carbon. This carbon content is due toremaining oxalic acid, which was used along with magnesium nitrate tosynthesize MgO—O, which was surprisingly not removed by washing andcalcination. The MgO—N sample displayed the highest Mg/O ratio, followedby MgO—O, then MgO-A.

TABLE 2 Elemental composition of synthesized MgO-based adsorbents Mg O CN Si Sample wt % wt % wt % wt % wt % MgO—A 47.2 52.8 — — — MgO—N 56.943.1 — — MgO—O 44.6 48.76 6.64 — — DETA-MgO—A 29.5 46.2 20.27 4.03 —PEI-MgO—A 17.4 42.2 27.2 13.2 —

Table 2 also shows the presence of carbon and nitrogen in theamine-functionalized samples (DETA-MgO-A and PEI-MgO-A), confirming thepresence of organic amines in/on MgO-A. The C and N contents are thehighest for the polymeric amine sample (PEI-MgO-A), in line with thefact that each PEI molecule contains many carbon and nitrogen atoms,compared to DETA-MgO-A since each DETA molecule contains three N atomsand four carbon atoms.

CO₂ Adsorption on the Synthesized MgO-Based Adsorbents

The adsorption of CO₂ on the synthesized MgO-based adsorbents wascarried out at ambient conditions (i.e., 30° C. and atmosphericpressure) using TGA. After the complete degassing of each adsorbent, CO₂was pumped at a flow rate of 100 mL/min and the total mass of the system(CO₂ plus adsorbent) was followed in time until a steady value wasapproached. The adsorption of CO₂ was first investigated on theunmodified MgO, prepared using the three different synthesis routes, toprobe the effect of the chemistry of the MgO synthesis solution on itsCO₂ adsorption capacity. The CO₂ adsorption results using MgO-A were thehighest of non-functionalized MgO adsorbents (shown in FIG. 3), whichdemonstrates the superiority of the magnesium oxide synthesized via theammonium hydroxide route (i.e., MgO-A). For instance, the adsorbedamount of CO₂ on MgO-A when the adsorption process approachedequilibrium is about 30 mg/g. In contrast, the adsorption capacity ofCO₂ on MgO—N and MgO—O are about 25 and 24 mg/g, respectively. The CO₂uptake capacities by the three MgO adsorbents are in a good agreementwith the results of the textural analysis shown in Table 1. For example,MgO-A displays the highest porosity (i.e., the highest surface area anda high pore volume) amongst the three unmodified MgO adsorbents. Thishigh porosity is likely to result in a higher adsorption capacity,according to some published reports. See Y.-D. Ding, G. Song, X. Zhu, R.Chen, Q. Liao, Synthesizing MgO with a high specific surface for carbondioxide adsorption, RSC Advances, 5 (2015) 30929-30935; and K. K. Han,Y. Zhou, W. G. Lin, J. H. Zhu, One-pot synthesis of foam-like magnesiaand its performance in CO₂ adsorption, Microporous and MesoporousMaterials, 169 (2013) 112-119, each incorporated herein by reference intheir entirety. However, porosity alone cannot explain the close CO₂adsorption capacity of MgO—N and MgO—O, despite that MgO—N possesses amuch higher porosity (e.g., the pore volume of MgO—N is about 9% higherthan that of MgO—O). Therefore, the variations in the crystallinity andmorphology of these adsorbents are likely to play a role. See J. M.Valverde, P. E. Sanchez-Jimenez, L. A. Perez-Maqueda, M. A. S.Quintanilla, J. Perez-Vaquero, Role of crystal structure on CO₂ captureby limestone derived CaO subjected tocarbonation/recarbonation/calcination cycles at Ca-looping conditions,Applied Energy, 125 (2014) 264-275; and G.-B. Elvira, G.-C. Francisco,S.-M. Victor, M.-L.R. Alberto, MgO-based adsorbents for CO₂ adsorption:Influence of structural and textural properties on the CO₂ adsorptionperformance, Journal of Environmental Sciences, 57 (2017) 418-428, eachincorporated herein by reference in their entirety.

Calcination temperature is one factor that might significantly alter thecharacteristics (e.g., crystallinity, morphology, porosity,compositions) of adsorbents and, accordingly, their adsorptioncapability. Thus, MgO-A was calcined at 300, 400, and 500° C. for 10 h.The CO₂ adsorption capacity results clearly demonstrate that calcinationat 400° C. produces a MgO adsorbent with the highest CO₂ adsorptioncapacity relative to the other two calcination temperatures. Increasingthe calcination temperature to 500° C. causes a significant reduction(nearly 30%) in the adsorption capacity, most likely due to asignificant level of MgO agglomeration at this temperature. See R. RajivGandhi, J. Suresh, S. Gown, D. S. Selvam, M. Sundrarajan, Effect ofCalcination Temperature on Surface Morphology of Ionic Liquid AssistedMgO Nanoparticles by Sol-Gel Method, Advanced Science Letters, 16 (2012)244-248, incorporated herein by reference in its entirety. A slightreduction (roughly 8%) in CO₂ adsorption capacity was encountered whenMgO-A was calcined at 300° C. Such a slight reduction in CO₂ adsorptioncapacity might be attributed to the incomplete conversion of magnesiumhydroxide to MgO at this calcination temperature. The presence of somemagnesium hydroxide in the MgO sample is likely to negatively impact itsCO₂ adsorption capacity. The very low CO₂ adsorption capacity of theas-prepared sample of Mg(OH)₂, which was not subjected to calcination,supports this assertion. Accordingly, the above results indicate thatcalcination temperature effects adsorbent properties, and that the mostadvantageous CO₂ adsorption properties are obtained when MgO-A iscalcined at about 400° C. See G. Song, Y.-D. Ding, X. Zhu, Q. Liao,Carbon dioxide adsorption characteristics of synthesized MgO withvarious porous structures achieved by varying calcination temperature,Colloids and Surfaces A: Physicochemical and Engineering Aspects, 470(2015) 39-45, incorporated herein by reference in its entirety.

Although MgO-A, calcined at 400° C., provides the highest CO₂ uptakecapacity among all unmodified MgO adsorbents, the adsorbed amount of CO₂is still unsatisfactory. To enhance CO₂ adsorption, MgO-A wasfunctionalized with amines having different features (DETA and PEI). Theadsorption of CO₂ on the DETA-functionalized MgO-A is also shown in FIG.3, which reveals the enhancement of CO₂ adsorption upon thefunctionalization of MgO-A with the DETA. For instance, the CO₂adsorption capacity increased by about 60% when MgO-A was functionalizedwith DETA (DETA-MgO-A CO₂ adsorption capacity of about 48 mg/g, comparedto about 30 mg/g for MgO-A). The higher CO₂ adsorption capacity ofDETA-MgO-A, despite having a 74% lower surface area than MgO-A, might beattributed to its higher pore size and higher CO₂ affinity of DETAmolecules. See S.-M. Hong, S. H. Kim, a. K. B. Lee, Adsorption of CarbonDioxide on 3-Aminopropyl-Triethoxysilane Modified Graphite Oxide, energy& fuels, 27 (2013) 3358-3363; and J. Pokhrel, N. Bhoria, S. Anastasiou,T. Tsoufis, D. Gournis, G. Romanos, G. N. Karanikolos, CO₂ adsorptionbehavior of amine-functionalized ZIF-8, graphene oxide, andZIF-8/graphene oxide composites under dry and wet conditions,Microporous and Mesoporous Materials, 267 (2018) 53-67, eachincorporated herein by reference in their entirety. Contrarily, thefunctionalization of MgO-A with PEI resulted in a 20% decrease in itsadsorption capacity (PEI-MgO-A CO₂ adsorption capacity of about 24 mg/g,compared to about 30 mg/g for MgO-A). Contrasting these results to thetextural properties of these adsorbents suggest that the enhancement ofCO₂ adsorption upon the attachment of PEI to MgO-A did not completelycompensate for the huge loss in the porosity (and, accordingly, thesurface area) of the support (i.e., MgO-A). See J. Pokhrel, N. Bhoria,S. Anastasiou, T. Tsoufis, D. Gournis, G. Romanos, G. N. Karanikolos,CO2 adsorption behavior of amine-functionalized ZIF-8, graphene oxide,and ZIF-8/graphene oxide composites under dry and wet conditions,Microporous and Mesoporous Materials, 267 (2018) 53-67, incorporatedherein by reference in their entirety.

Regeneration of DETA-MgO-A Adsorbent

The regenerability of DETA-MgO was evaluated by conducting 4 consecutiveadsorption cycles (FIG. 4). Extended cycles of adsorption-regenerationcan also be carried out with negligible loss in the adsorption capacityof DETA-MgO. The adsorption of CO₂ took place from a pure CO₂ gas streamflowing at 100 mL/min. The adsorption temperature and pressure werefixed at 30° C. and 1 atm. The regeneration was conducted using N₂ gasstream flowing at 100 mL/min. The regeneration temperature and pressuretime were 120° C. and 1 atm, respectively. DETA-MgO was fullyregenerated within less than 20 min of contact with the N₂ gas stream atthe aforementioned regeneration conditions.

Thus, the synthesized MgO-based adsorbents herein are promisingmaterials for CO₂ capture at ambient conditions. MgO can be produced viadifferent synthesis routes; each route influencing the characteristicsof the obtained MgO and, accordingly, its CO₂ adsorption capability.Among the three MgO synthesis routes tested, the ammonium hydroxideroute offers the highest CO₂ adsorption performance. MgO synthesizedusing this route (MgO-A) displayed the highest surface area and thehighest CO₂ adsorption capacity. In addition to the chemistry of the MgOsynthesis solution, the calcination temperature of the as-preparedmaterial is also a factor to adsorbent properties. Calcination at 400°C. resulted in the highest CO₂ adsorption. On the other hand, highercalcination temperatures (e.g., 500° C.) caused agglomeration and a lowcalcination temperature (e.g., 300° C.) resulted in incomplete formationof MgO, each causing a lower adsorption capacity of the preparedmaterial. Functionalization of MgO with polyamines resulted in anenhancement of CO₂ adsorption. Although the surface area is a keyfactor, it is not the only one. Pore size, amine-CO₂ interaction (i.e.,amine affinity for CO₂) and adsorbent morphology are also importantfactors. The adsorbents are completely and repeatedly regenerable,making them good candidates for commercial CO₂ capture applications,with low-energy consumption during the mild regeneration processes.

The invention claimed is:
 1. An aminated magnesium oxide adsorbent,comprising: a magnesium oxide matrix having disordered mesopores and aBET surface area of 320 to 380 m²/g; and a polyamine selected from thegroup consisting of an ethyleneamine having a molecular weight of up to450 g/mol and a polyethylene imine having a number average molecularweight of greater than 500 g/mol and up to 20,000 g/mol; wherein thepolyamine is impregnated within the disordered mesopores of themagnesium oxide matrix.
 2. The aminated magnesium oxide adsorbent ofclaim 1, wherein the magnesium oxide matrix is prepared fromprecipitation of magnesium hydroxide from a solution of a magnesium saltand ammonium hydroxide, followed by calcination of the magnesiumhydroxide at 350 to 450° C.
 3. The aminated magnesium oxide adsorbent ofclaim 1, wherein the magnesium oxide matrix has an average pore volumeof 0.3 to 0.5 cm³/g and an average pore size of 3 to 6 nm.
 4. Theaminated magnesium oxide adsorbent of claim 1, wherein the magnesiumoxide matrix consists essentially of magnesium oxide.
 5. The aminatedmagnesium oxide adsorbent of claim 1, wherein the polyamine is theethyleneamine.
 6. The aminated magnesium oxide adsorbent of claim 5,wherein the ethyleneamine is at least one selected from the groupconsisting of diethylenetriatnine, triethylentetramine,tetraethylenepentamine, pentaethylene hexamine, and hexaethyleneheptamine.
 7. The aminated magnesium oxide adsorbent of claim 5, whereinthe ethyleneamine is diethylenetriamine.
 8. The aminated magnesium oxideadsorbent of claim 1, wherein the polyamine is the polyethylene imine,and the polyethylene imine is a linear polyethylene imine.
 9. Theaminated magnesium oxide adsorbent of claim 1, which has a magnesiumcontent of 15 to 31 wt. %, an oxygen content of 40 to 50 wt. %, a carboncontent of 17.5 to 30 wt. %, and a nitrogen content of 2 to 16 wt. %,each based on a total weight of the aminated magnesium oxide adsorbent.10. The aminated magnesium oxide adsorbent of claim 1, which has a BETsurface area of 40 to 98 m²/g.
 11. The aminated magnesium oxideadsorbent of claim 1, which has an average pore size of 7 to 11 nm. 12.The aminated magnesium oxide adsorbent of claim 1, which has an averagepore volume of 0.05 to 0.3 cm³/g.
 13. The aminated magnesium oxideadsorbent of claim 1, which is crystalline by XRD.
 14. The aminatedmagnesium oxide adsorbent of claim 1, which has a CO₂ uptake capacity of24 to 60 mg CO₂ per 1 g of the aminated magnesium oxide adsorbent at 30°C. and 1 atm.
 15. A method of making the aminated magnesium oxideadsorbent of claim 1, the method comprising: precipitating magnesiumhydroxide from a solution of a magnesium salt and ammonium hydroxide;calcining the magnesium hydroxide at 350 to 450° C. for 6 to 24 hours toform the magnesium oxide matrix having disordered mesopores; and wetimpregnating the magnesium oxide matrix with an alcoholic solution ofthe polyamine.
 16. The method of claim 15, wherein a molar ratio of theammonium hydroxide to the magnesium salt in the solution is 2:1 to 9:1,and wherein the precipitating is performed by heating the solution to 50to 70° C. for 3 to 10 hours, followed by stirring the solution at 20 to30° C. for 12 to 48 hours.
 17. The method of claim 15, wherein a weightratio of the polyamine to the magnesium oxide matrix for the wetimpregnating is 1:1 to 1:3.
 18. A method of capturing CO₂ from a gasmixture comprising CO₂ and at least one other gas selected from thegroup consisting of hydrogen, oxygen, nitrogen, methane, and carbonmonoxide, the method comprising: contacting the gas mixture with theaminated magnesium oxide adsorbent of claim 1 to adsorb at least aportion of the CO₂ into the aminated magnesium oxide adsorbent, therebyforming a loaded aminated magnesium oxide adsorbent and a gas streamdepleted in CO₂ compared to the gas mixture.
 19. The method of claim 18,wherein the gas mixture is a pre-combustion gas mixture comprising 15 to50 vol. % of CO₂, based on a total volume of the gas mixture.
 20. Themethod of claim 18, wherein the gas mixture is a post-combustion gasmixture comprising 5 to 15 vol. % of CO₂, based on a total volume of thegas mixture.